Ok. You can’t do that by punching-in nested events, cause nested events are async, which means they always start at their beginning. You can use nested events but you’ll have to play them from the beginning, and automate the volume. Note that the synchronisation between tracks will never be checked anymore, so it can be somewhat risky ; it’s safer to use synced instruments. Either ways, the problem is the volume automation (or AHDSR). In fact you’ll be facing the problem that FMOD lacks an essential feature, which I asked for almost 2 years ago, and which is the most asked feature on this forum :
and
You’ll find some workarounds on those links, though.