like a magic item or a feat or something? (without homebrew)

(short answer:no)

Without homebrew, the answer would be no (unless there is some super obscure item from a module I have not read).

good to know,thanks.

Ring of Spellstoring used by your Familiar or by another player who doesn‘t use spells.

In fact in the DMG it suggests that the single concentration spell at a time rule is one that shouldn't be violated even by homebrew.

the answer to your question is - no, there is no way to cast two or more spells at the same time...and concentration has nothing to do with the casting of a spell.

if you were to ask the question 'is there a way to maintain concentration on two or more spells at the same time?'  the answer is also no, although i've seen many players do that because they just didn't understand how concentration works or just weren't paying attention to spell descriptions.

Artificers with spell storing item, or Chronurgist Wizards with their spell-freezing ability. 

Quote from LeviRocks >>

Artificers with spell storing item, or Chronurgist Wizards with their spell-freezing ability. 

are you saying either of those allow you to cast two spells at the same time, or allow you to concentrate on two spells at the same time? if so, can you point to exactly what you're looking at?

thanks

Spell-storing Item lets someone else concentrate on one of your spells for you: “While holding the object, a creature can take an action to produce the spell’s effect from it, using your spellcasting ability modifier. If the spell requires concentration, the creature must concentrate.”

Chronurgy magic can be read the same way: “A creature holding the bead can use its action to release the spell within, whereupon the bead disappears. The spell uses your spell attack bonus and save DC, and the spell treats the creature who released it as the caster for all other purposes.”

However, I don’t think the Chronurgy one actually works by RAW. Unlike the artificer, the Chronurgist does actually have to cast the spell, which brings into play normal concentration rules. Since it’s frozen in time, I’m on board with the idea of it maintaining its duration while in the bead, but as soon as someone else releases the spell, it would immediately end unless the original caster is still concentrating. There’s no language to support “transference” of concentration from the first caster to the user of the bead.

so i believe the answer is still no...you can not cast two spells at the same time, and you can not concentrate on two spells at the same time.

saying you can use one of those items/abilities to concentrate on two spells at once is like saying you can loan your extra dagger to someone else and now you're attacking twice at the same time.  sure...two of your daggers can be used in attacks each round.

Quote from SagaTympana >>

Spell-storing Item lets someone else concentrate on one of your spells for you: “While holding the object, a creature can take an action to produce the spell’s effect from it, using your spellcasting ability modifier. If the spell requires concentration, the creature must concentrate.”

Chronurgy magic can be read the same way: “A creature holding the bead can use its action to release the spell within, whereupon the bead disappears. The spell uses your spell attack bonus and save DC, and the spell treats the creature who released it as the caster for all other purposes.”

However, I don’t think the Chronurgy one actually works by RAW. Unlike the artificer, the Chronurgist does actually have to cast the spell, which brings into play normal concentration rules. Since it’s frozen in time, I’m on board with the idea of it maintaining its duration while in the bead, but as soon as someone else releases the spell, it would immediately end unless the original caster is still concentrating. There’s no language to support “transference” of concentration from the first caster to the user of the bead.

 The Arcane Abeyance says, "The spell uses your spell attack bonus and save DC, and the spell treats the creature who released it as the caster for all other purposes."

This means that essentially, the person who released the spell concentrates on it, because they count as the person who cast it. 

So, long story short, there is no ability that lets you cast 2 concentration spells at the same time and concentrate on both of them at the same time, but there are a few abilities that let you give the concentration to other people. 

Quote from LeviRocks >>

 So, long story short, there is no ability that lets you cast 2 concentration spells at the same time and concentrate on both of them at the same time, but there are a few abilities that let you give the concentration to other people. 

 eh?????? you're not transferring concentration from one character to another. the character triggering the spell is using their own ability to concentration.   there is no transfer or giving of concentration.

Quote from HeathSmith >>

Quote from LeviRocks >>

 So, long story short, there is no ability that lets you cast 2 concentration spells at the same time and concentrate on both of them at the same time, but there are a few abilities that let you give the concentration to other people. 

 eh?????? you're not transferring concentration from one character to another. the character triggering the spell is using their own ability to concentration.   there is no transfer or giving of concentration.

 

 Yeah, the person who triggers the spell does concentrate on it, but this allows you to have the party paladin concentrate on Suggestion for you, or the Beast Master Animal Companion to concentrate on Flaming Sphere for you.

You're still expending spell slots to do this, but you can have 2 concentration spells that come from your Spellcasting Ability on at the same time. 

You are transferring concentration of one of your spells from you to an ally. 

There may also be features that allow you to cast something in a special way that does not require concentration - for example, the Variant Ranger gets a few free daily casts of Hunter's Mark that do not require concentration to maintain.

There are at least two items that does let you do this; 

• Blackrazor, but the sword is the caster, it applies only to Haste, it chooses when to cast it, and it maintains concentration.  
• Shield of the Hidden Lord has an aura of dread, also not under the player's ability.

So both items are legendary, so this isn't a trivial power to hand off, and in both cases it is very narrow on what power it applies to, and in both cases, neither is in the control of the player.

There is no way to cast 2 spells at the same time.

You can cast two spells in the same turn, more even, in certain circumstances and situations. Such as using a bonus action spell and a cantrip, using action surge, etc. Some effects let you double the effect of a spell as if casting it twice, such as Twinned Spell metamagic. But that's it, and again, not "at same time".

There are no rules for letting you maintain concentration on two spells at once. There are features that can let you cast a spell without needing it to have concentration, some spells can let you avoid having to maintain concentration to sustain the spell if cast at higher levels (such as Major Image). There are features like Spell-Storing Item (Artificer) and Arcane Abeyance (Wizard - Chronurgy Magic) and magic items like Ring of Spell Storing and Spell Gems which all let another creature bear the burden of concentration for a spell you cast but the spell basically becomes theirs. However, none of these remove restrictions : you cannot concentrate, yourself, one more than 1 spell at a time.

There was a feat in the Tal'dorei Campaign Guide - which is the not-official precursor to Explorer's Guide to Wildemount - based on rules Matt made for the Critical Role show, that did allow this but with heavy restrictions. Given that the majority of the Campaign 2 cast have spellcasters and none of them took the feat I'm guessing Matt did not allow this feat the second time around.

Concentration is certainly annoying, but the balance system is heavily reliant on it and so any changes to it will have very significant changes to the whole of spellcasting. Even low-level spells can become so much more dangerous by allowing a single caster to combo spells normally requiring concentration, like Hold Person and Cloud of Daggers. It's easier to protect that one caster to maintain it than trying to protect two (normal), and it means that one caster can, at low levels, out-damage mid-level martial characters consistently.

Be Niv'Mizzet from Guildmaster's Guide to Ravnica.

Quote from CharlesThePlant >>

Be Niv'Mizzet from Guildmaster's Guide to Ravnica.

 
As a CR 26 character that cannot be achieved. The highest CR any player creature can become is 20, via True Polymorph.

Niv'Mizzet is just shy of Tiamat in power and she's an actual Dragon God.

Quote from Nthal >>

There are at least two items that does let you do this; 

• Blackrazor, but the sword is the caster, it applies only to Haste, it chooses when to cast it, and it maintains concentration.  
• Shield of the Hidden Lord has an aura of dread, also not under the player's ability.

So both items are legendary, so this isn't a trivial power to hand off, and in both cases it is very narrow on what power it applies to, and in both cases, neither is in the control of the player.

 There are potions that allow this as well, like the potion of diminution, and the potion of growth. 

Quote from Cyb3rM1nd >>

Quote from CharlesThePlant >>

Be Niv'Mizzet from Guildmaster's Guide to Ravnica.

 
As a CR 26 character that cannot be achieved. The highest CR any player creature can become is 20, via True Polymorph.

Niv'Mizzet is just shy of Tiamat in power and she's an actual Dragon God.

 Yeah not an actual serious suggestion Haha