Do multiple sources of temporary hit points combine, or do you only get one set?
For example, say you cast armor of Agathys and false life on yourself. Do you have two separate pools of temporary hit points, or do you have to pick one of the two? By separate pools, I mean that you have 5 temp HP from AoA and 8 temp HP from FL. If you take 10 damage, can you pick one of the two pools of temp HP and take the leftover damage to your real HP, but still have the other pool to protect against a second attack?
I ask because a warlock player in my game insists that she gets both pools, but I think it should be either-or.
How do multiple sources of temp HP interact with one another?