Whether they stack or not depends on how a fallacy is cleared up!
If they do stack the stacks don't do much.
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect--such as the highest bonus--from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.
In regards to this statement. It can be said that an equivocation exists as to the meaning (or duality of meanings) of 'effect.' Said plainly, effect could have 2 different meanings
Case 1: A spell's effect could be tied to the spells existence, if the spell is not in effect it simply isn't there.
Case 2: Or a spell's effect (what the spell does) can be separate from the existence of the spell itself. in which case, should two of the same spells overlap, only one effect would take effect, but that doesn't mean the other spell fizzled out of existence, This would allow scenarios such as the following: if wizard 1 casts haste on a fighter, then after 5 turns wizard 2 also casts haste on the figher; only the latter haste's effect (both being equally potent) would apply, both mages could still maintain concentration, and when mage 2 (who's spell is in effect) loses concentration, it falls back to the first mage's spell, who is still concentrating on it, which is again taking effect.
So which definition of 'effect' is accurate? The wording within the 'combining magical effects' rules suggest the second definition is probably the correct one. The mention of spell durations overlapping suggests multiple spells can exist even if only one was in effect, From the first quote I bolded two separate instances in which the ruleset mentioned the overlapping of durations. The first sentence is in reference to different spells which have different effects, but the second sentence refers to the same spell cast multiple times. Reiterating what I said, the fact that the most recent effect applies while their durations overlap suggests that both spells are still ongoing; the second spell does not override the first spell; however, the second 'spell effect' does override the first 'spell effect.'
That said, one can still make an argument that either could be the case, because as said, this is an equivocation. The fact that the 'combining magical effects' ruleset suggests one definition is likely right, doesn't confirm that it is right. You can't ever determine what is meant in cases of fallacy without the publisher expressing what they intend the meaning to be. So then lets answer your question for both cases: given the first case, Bruno Souza already answered it. One spell would overwrite the other, there is nothing more to discuss.
For the second case, layers of TP are in fact being applied to the creature. However, those layers of TP are going to do little (probably nothing) for you. Lets read some of true polymorph:
The transformation lasts for the duration, or until the target drops to 0 hit points or dies...
Later it says
The target assumes the hit points of its new form, and when it reverts to its normal form, the creature returns to the number of hit points it had before it transformed.
You don't return to to the hit points you had before you transformed until you revert back to your normal form. However many layers of TP you have on you, when you drop to 0 hit points, your transformation reverts to the last but remains at 0 hit points, thus it reverts to the tranformation before that, you only return to how many hit points you had before transformation when you return to your normal, untransformed form. The last layer that ended would have been the first one you transformed into, so the hit points you return to is the amount you had in your normal form, before your first transformation.
So to answer your question, depending on the meaning of 'effect', the answer is different. If a spell's effect was tied to the spell's existence TP wouldn't form layers; a second cast dispel the first, or fizzle if it was less potent than the first. If an effect was divorced from the spell's existence, then layers of TP would form. But if you dropped to 0 hp every layer would be gone, so the layers wouldn't make much difference.
Sorry for the long post I'm not good at not being thorough. Hope this was a satisfactory answer.