Jack Simth said:
It depends on a simple ruling the DM makes...
It's a fraction more complex, unfortunately.
There's disagreement as to what 'overlapping' THP actually means.
Same Effect More than Once in Different Strengths: In cases when two or more identical spells are operating in the same area or on the same target, but at different strengths, only the best one applies.
Some will suggest that if someone is the target of three False Life spells, for 20, 30, and 40 temp hit points respectively, that the 40 THP spell the best of identical spells operating at different strengths.
If he has 60 hit points of his own, then he temporarily has 100 hit points - 60, plus 40 temporary hit points from the best of the three spells.
If he gets hit for 15 damage, that damage comes off the temporary hit points supplied by the operative False Life spell first. The THP of the three spells is now 20, 30, and 25 respectively. The active spell is no longer the best of the three identical spells, and goes quiescent; the 30 THP spell is now active. The character's effective hit point total is 90 - 60, plus 30 from the active spell.
If he gets hit for another 15 damage, his three spells now rate 20, 15, and 25 respectively. The active spell goes quiescent, and the 25 THP spell takes over, leaving him with an effective total of 85.
Now, what's the difference between this interpretation of overlapping, where he has a total of 85 hit points, but can take 120 more hit points before collapsing, and if the spells stacks, where he'd have a total of 120 hit points, and could take 120 hit points before collapsing? In the first case, he's subject to Power Word: Kill, and in the second, he isn't.
... that's about the only difference between 'stack' and 'don't stack' under that reading of overlapping.
In order to make overlapping THP behave as though they're
actually overlapping, I'd personally read it that if two False Life spells have identical totals, they're
both active (though the character's hit point total is only raised once, not twice), and thus any damage removes THP from both spells.
So in the above example, when the three spells have 20, 30, and 40 THP respectively, and the character is hit for 15 damage, the first 10 of that comes off the 40 THP spell only. The remaining five damage, however, comes off both the 30 THP spell
and the originally-40 THP spell, since both of these now have 30 THP left and are both operating in identical strength.
Following that hit, the character has 60 hit points, plus three False Life spells supplying 20, 25, and 25 hit points respectively - a total of 85 (exactly what we'd expect, after a total of 100 gets hit for 15). After a second hit for 15 damage, he has 60 hit points, plus three False Life spells each supplying 10 hit points - a total of 70 (again, what we'd expect, after a total of 85 gets hit for 15).
-Hyp.