So I'm aware that resistances don't stack, if a tiefling has a ring of fire resistance, they don't take 1/4th fire damage.
What I'm confused on is if 2 different resistances can.
For example, if you had resistance to damage from spells and resistance to fire damage, would you take 1/4th damage from a firebolt or fireball? Or say you had resistance to piercing damage and also damage from traps, and your hit by an arrow trap, is that 1/4th damage or half?
You can only resist damage once. If you resist spell damage and resist fire damage, you take half damage from fireball (you can half the half damage from save, that is different).
As far as I know, there is no such thing as resistance to traps or spells. Resistance is resistance to a damage type. A given damage is only of one type, so there can be only one type of resistance that applies.
If you homebrew other types of resistance, you will have to manage the overlap, but I would advise against it, it would really complicate the game.
This isn't always true. The Dungeon Delver feat grants resistance to damage from traps, and the Oath of the Ancients's Aura of Warding grants resistance to spell damage. Resistance to nonmagical attacks is also common among monsters and isn't type-based.
In fact the section of the rules you linked to contains an example where two instances of resistance apply. It also contains the answer to the question: no matter how many sources of resistance apply, the damage only gets halved once. The only way to reduce damage further is through other mechanics, like the Rogue's Uncanny Dodge or the Heavy Armor Master feat.
So I'm aware that resistances don't stack, if a tiefling has a ring of fire resistance, they don't take 1/4th fire damage.
What I'm confused on is if 2 different resistances can.
For example, if you had resistance to damage from spells and resistance to fire damage, would you take 1/4th damage from a firebolt or fireball? Or say you had resistance to piercing damage and also damage from traps, and your hit by an arrow trap, is that 1/4th damage or half?
Resistance to spells isn't damage resistance, it's advantage on saves. I don't know where you found resistance to traps, but I'd guess it's the same (advantage on saves, and/or disadvantage on traps to hit you). Regardless, Xanathar's clears this up: damage resistance can only apply to a given amount of damage once, no matter what. The PHB also covers this (since monsters can easily be resistant to all non-magical and a damage type), clarifying that it's 1/2, not 1/4.
On the other hand, if a rogue with uncanny dodge wearing full plate with the heavy armor master feat and piercing resistance and piercing vulnerability is shot in the face with an arrow, that all stacks. The rogue will take this much damage, if the attacker delivered, say, 40 piercing damage:
Uncanny Dodge happens on hit, before the damage is even rolled, so it happens first. 40 becomes 20.
Heavy Armor Master happens on damage, but has precedence over resistance. 20 becomes 17.
Guys it doesn't matter if you can resist spell or trap damage (which you can gor both: dungeon delves feat and oath of ancients aura). The only thing that matters is that multiple instances of resistance don't stack.
Damage is only halved by resistance once no matter how many times or ways you resist is.
If you succeed the save for fireball cast by a magic trap, while being within 10 feet of an oath of ancients paladin, and are a tiefling with dungeon delver and cast absorb elements as a reaction, you take 1/4th the damage rolled (1/2 for save, 1/2 for resistance).
So I'm aware that resistances don't stack, if a tiefling has a ring of fire resistance, they don't take 1/4th fire damage.
What I'm confused on is if 2 different resistances can.
For example, if you had resistance to damage from spells and resistance to fire damage, would you take 1/4th damage from a firebolt or fireball? Or say you had resistance to piercing damage and also damage from traps, and your hit by an arrow trap, is that 1/4th damage or half?
You can only resist damage once. If you resist spell damage and resist fire damage, you take half damage from fireball (you can half the half damage from save, that is different).
This isn't always true. The Dungeon Delver feat grants resistance to damage from traps, and the Oath of the Ancients's Aura of Warding grants resistance to spell damage. Resistance to nonmagical attacks is also common among monsters and isn't type-based.
In fact the section of the rules you linked to contains an example where two instances of resistance apply. It also contains the answer to the question: no matter how many sources of resistance apply, the damage only gets halved once. The only way to reduce damage further is through other mechanics, like the Rogue's Uncanny Dodge or the Heavy Armor Master feat.
The Forum Infestation (TM)
Resistance to spells isn't damage resistance, it's advantage on saves. I don't know where you found resistance to traps, but I'd guess it's the same (advantage on saves, and/or disadvantage on traps to hit you). Regardless, Xanathar's clears this up: damage resistance can only apply to a given amount of damage once, no matter what. The PHB also covers this (since monsters can easily be resistant to all non-magical and a damage type), clarifying that it's 1/2, not 1/4.
On the other hand, if a rogue with uncanny dodge wearing full plate with the heavy armor master feat and piercing resistance and piercing vulnerability is shot in the face with an arrow, that all stacks. The rogue will take this much damage, if the attacker delivered, say, 40 piercing damage:
Any questions?
Guys it doesn't matter if you can resist spell or trap damage (which you can gor both: dungeon delves feat and oath of ancients aura). The only thing that matters is that multiple instances of resistance don't stack.
Damage is only halved by resistance once no matter how many times or ways you resist is.
If you succeed the save for fireball cast by a magic trap, while being within 10 feet of an oath of ancients paladin, and are a tiefling with dungeon delver and cast absorb elements as a reaction, you take 1/4th the damage rolled (1/2 for save, 1/2 for resistance).