The Hat of Disguise is a magic item allows you to cast Disguise Self at will. Nondetection allows you to hide a target from divination magic. 

If you have cast disguise self on yourself, you will radiate an aura of illusion magic to anyone focusing on you with Detect Magic. Similarly, the Hat of Disguise would be detected by this divination as an object that bears magic. So, if you cast Nondetection on yourself, you can hide the aura around you from this divination. However, Nondetection only works on a single target. 

My question is, while wearing the hat do you and the hat count as a single target or as two separate targets? That is to say, if I wanted to make my disguise immune to the effects of divination magic, would I need to cast Nondetection once or twice ? 

Yeah, IMO you and all the stuff you're wearing are one target. After all, you don't have to cast nondetection separately on your boots, your armor, your clothes, your rings, etc...

I would say that the protection offered by Nondetection does apply to whatever the target is wearing as well.

However, Nondetection does not do what you think it does. Here’s the effect: “The target can''t be targeted by any divination magic or perceived through magical scrying sensors.” Detect Magic targets the caster, not an object being examined, and it is not a scrying sensor. Therefore, Nondetection doesn’t do anything to hide its target from Detect Magic.

There is a spell that does this, however: Nystul’s Magic Aura. In this case, I would treat the target and its magic items separately.

Quote from SagaTympana >>

However, Nondetection does not do what you think it does. Here’s the effect: “The target can''t be targeted by any divination magic or perceived through magical scrying sensors.” Detect Magic targets the caster, not an object being examined, and it is not a scrying sensor. Therefore, Nondetection doesn’t do anything to hide its target from Detect Magic.

I see. I overlooked that detail, good point. I was moreso considering this combination for an NPC in a campaign I am running rather than a player character. This NPC has Nondetection prepared but not Nystul's Magic Aura. I suppose I could simply change the stat block, its not as if Nystul's is much more powerful than Nondetection. 

Thanks for the help. 

Non detection can be cast on a creature. Detect magic is from the school of divination. 

non detection prevents anything from the school of divination. 

explain why this wouldn’t work 

Hat of disguise + Periapt of proof against detection and location= darn good combo …

The exact words of Nondetection: "For the duration, you hide a target that you touch from divination magic. The target can be a willing creature or a place or an object no larger than 10 feet in any dimension. The target can't be targeted by any divination magic or perceived through magical scrying sensors."

Detect Magic would be targeting the caster, not the one benefiting from Nondetection. Therefore, rules as written, the one casting Detect Magic would sense the abjuration magic coming from Nondetection as well as the illusion magic from the hat. However, if the caster attempted to cast Identify on the one with Nondetection to learn what the abjuration spell or illusion spell was it wouldn't work since Identify would be directly targeting them.

I don't see where I said or implied the Nondection spell would not extend to items the target is carrying. I said that as the spells are written the Detect Magic spell would detect the Nondetection spell.

Of course, as always, DMs are free to change rules as they see fit. I myself would probably rule that Nondetection beats Detect Magic.

Quote from SamCornick >>

I myself would probably rule that Nondetection beats Detect Magic.

 This is a good call, since it does RAW.

Can you please elaborate? As stated, as written Nondetection blocks targeted divination spells. Since Detect Magic would be not targeting  the benefactor of Nondetection, how is it RAW that Nondetection beats Detect Magic.

Would you agree that detect magic is divination magic?

It is indeed a nontargeted divination spell. I pointed out multiple times that the nontargeted aspect of Detect Magic is what would allow it to pick up on Nondectection.

If I cast nondetection on myself, I am hidden from divination magic, per the spell's text.

Per the spell's text "The target can't be targeted by any divination magic or perceived through magical scrying sensors." Yes, it feels like splitting hairs but this distinction of targeting versus nontargeting is what would allow Detect Magic to work. But I feel like we've gone back and forth enough on this. Of course you are free to interpret things your way as I can mine. I hope you have a pleasant weekend!

Both of these statements are true because the spell tells us so:

For the duration, you hide a target that you touch from divination magic.

and

The target can't be targeted by any divination magic or perceived through magical scrying sensors.

I hope you also have a pleasant weekend.

Quote from SamCornick >>

I don't see where I said or implied the Nondection spell would not extend to items the target is carrying. I said that as the spells are written the Detect Magic spell would detect the Nondetection spell.

Yea the wording of Nondetection is problematic and your argument possibly is the RAW answer.
But by my count there are 35 Divination spells and 19 of those have a range of "Self" (as Detect Magic). Of the remaining 16 spells almost half are buff spells (that you can't benefit from) and then a few harmful spells that would be blocked. All in all it would make Nondetection basically useless, especially as "scrying sensor" is equally poorly defined.

Of course there is the fact that Nondetection says divination magic not divination spells that might make a difference. And as we've seen with the ruling about Twinned Spell the designers considers things affected by a spell effect to be targets of the spell too. So I'd likely come to the conclusion that Nondetection works against Detect Magic (and most all divination spells).

Detect Magic beats Nondetection. 

If you want to protect from Detect Magic use Nystul’s Magic Aura / Arcanist's Magic Aura

Nondetection is there to protect from targeted divination such as harmful ones or scrying. Magic Aura is there to protect from Detect Magic. 

I think this conversation is getting too caught up in the targeting issue. Targeting is only part of what nondetection protects against. First and foremost, nondetection hides its target from divination magic, which is the very first thing the spell description says. Detect magic is divination magic, therefore the target of nondetection is hidden from detect magic. The comparison does not need to be any more complicated than that. Avoiding detection is literally in the name of the spell. It is the one purpose of the spell. The fact that Nystul's magic aura also has a limited version of this effect has no bearing on whether nondetection does what it says it does.

A creature under the effect of nondetection can't be targeted by any divination magic or perceived through magical scrying sensors, including Detect magic  and any other spells from the divination school.

Yep. While under the effect of nondetection, you cannot be targeted by detect magic. The thing is, detect magic only targets whoever it's cast on. So all that really means is that you can't have detect magic cast on you.

"Magical scrying sensors" refers to spells like scrying, arcane eye, divination, and debatably alarm and find familiar.