cos think bout it, the PSI will only increase as the air is compressed further, when the air leaves the turbo into the the intercooler and on the way to the intake plenum it stays at the same compression level it already is, just theres more of it trying to get in, the air from another turbo cant compress the air from the first turbo if its at the same PSI. If however one turbo was running 10 psi and one was running 20psi i think, not sure though, you would end up with a total of 15Psi in the motor ??
edit: just thinking as well pressure is relative, 15 psi in a 10lt container is a lot less air than 15 psi in a 20 lt container. so the turbo isn't sending out a static '12 psi' its pushing air until the pipes its filling reach 12psi, both turbos know when that pressure is reached and stop, i guess 2 turbos work half as hard as 1 on its own ?
to answer the first question that doesnt mean theyre both pushing six, cos theyre both a part in the same system, the psi in that system is always the same for both turbos.
the way your thinking about it basically is that both turbo's are sealed off, reach 12 psi in a certain volume. and then both release they're pressurised air into a container the same size as each of the individual originals, but even then it wouldnt create 24 psi cos unless u have 24psi behind it it wont squeeze in