Do Temp HP effects stack if both occur simultaneously?

Question

So, I have a player who through some multiclassing and the like, gained two abilities that both said that when he kills someone he gains temporary HP equal to half his level + his Charisma mod. Does he gain both these boosts when he kills someone?

I know that temporary HP doesn't stack, but this happens at the same time. The abilities are specifically Dark One's Blessing and a homebrew one that has the exact same wording, but a different name. I didn't think it through on creation how these two abilities would interact with each other.

Answer 1

The timing doesn't matter: temporary HP does not stack

It doesn't matter how many different abilities the character triggers simultaneously that grant them temporary HP, they cannot be added together.

Temporary hit points aren’t cumulative. If you have temporary hit points and receive more of them, you don’t add them together, unless a game feature says you can. Instead, you decide which temporary hit points to keep. (XGE)

The fact that this rule says that they are not cumulative ever means that timing is essentially a non-issue here. You can never add temporary hit points together unless an ability says you can, even if you were able to get them at the exact same time.

D&D doesn't work when you resolve some things simultaneously

In D&D, things don't really happen at the same time. Built into the game is the unspoken assumption that things resolve one by one, in some order. In fact, you can see this plainly by the issue that you have this question. The rule for temporary HP was built around the assumption that things resolve in an order and not all at once.

A good example of how things don't work when you try to resolve things simultaneously is that there is simply no way to resolve things at times. For example, what happens when someone casts fireball on a person at the same time they are casting resist fire on themselves? Since one spell affects the other, there is no way to resolve them at the same time. Thus, the only way to do it is one at a time. Many things don't matter and can be hand waived away, but as you discovered in your question, some things are built around the idea that things are resolved one at a time and break if you try to run them the other way.¹

That is why I play with (and highly recommend other tables do as well) the optional rule on simultaneous effects from Xanathar's Guide to Everything p. 77 (IMO one of the best rules they have ever added to the game).

Most effects in the game happen in succession, following an order set by the rules or the DM. In rare cases, effects can happen at the same time, especially at the start or end of a creature’s turn. If two or more things happen at the same time on a character or monster’s turn, the person at the game table — whether player or DM — who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character’s turn, the player decides which of the two effects happens first.

So, the order is decided by whoever controls the character whose turn it is when multiple things trigger. They then are resolved one at a time.

In the case of temporary HP it doesn't matter regardless because the player can always choose to keep their current temp HP or swap it for new ones. In a theoretical case where the two abilities were different this would allow a character to swap their current temporary HP for a greater amount for example. However, the timing would still not matter. If things happen at the same time or one after another doesn't affect how the rules adjudicate it. They can never choose to benefit from both.

Your specific case

So, this player would get temporary hit points equal to CHA mod + warlock level (note not half the level as you stated in your question) when they reduce a creature to 0 HP. Since both trigger at the same time and give the same number of hit points, it doesn't matter which they choose. They will receive CHA mod + warlock level temporary hit points and the other ability will essentially have no effect.

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^{1 - thank you to @thedarkwanderer for coming up with this example and working out the idea with me.}

Answer 2

Yes, unless you play with the optional rule in Xanathar's Guide to Everything

If you are playing the game without the optional rule regarding simultaneous resolution, things that happen at the same time happen at the same time. Since the only reason you can't stack temporary hp is:

If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones. For example, if a spell grants you 12 temporary hit points when you already have 10, you can have 12 or 10, not 22.

(PHB pg 198)

and since you don't have temporary hit points when receiving either of the on-kill buffs, you don't have to decide between them and can just keep both.

If, however, you are playing with the optional rule on page 77 of XGE:

In rare cases, effects can happen at the same time, especially at the start or end of a creature's turn. If two or more things happen at the same time on a character or monster's turn, the person at the game table - whether player or DM - who controls that creature decides the order in which those things happen.

you can no longer do this, because it is then impossible for two things to ever happen at the same time (except for initiative tie-breaking choices)-- the person controlling the character whose turn it is decides the order that things happen in. In this case, the active character's controller would decide what order the PC resolves their abilities in, which would not matter ordinarily as the PC would then decide to keep the temporary hp from either roll.

Note also that it's not relevant that the effects in question provide an identical buff; the DMG errata specifies that only effects with the same name, not effects with the same effect, fail to combine due to the general 'nothing's allowed to self-stack ever' rules:

Chapter 8 Combining Game Effects (p. 252). This is a new subsection at the end of the “Combat” section: Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap. For example, if a target is ignited by a fire elemental’s Fire Form trait, the ongoing fire damage doesn’t increase if the burning target is subjected to that trait again. Game features include spells, class features, feats, racial traits, monster abilities, and magic items. See the related rule in the “Combining Magical Effects” section of chapter 10 in the Player’s Handbook.

and the chapter 10 passage mentioned addresses only spells, which these effects aren't.