I think you are confused
Sounds to me like you are a little confused on the turbocharger designations.
A T-88 is not bigger than a T-72!
ok here is how it works.....the numbers are meaningless unless you know what context you are comparing them with.
a T-88 is a mitsubishi turbo, and all it means it that it is using a TD08 turbine housing and a TD08 compressor housing. It is their big dog....but not all T-88s are equal. It is just a family designation. Same goes for a T-78 , it is a Td07 exhaust with a TD08 compressor housing. although for all intents and purposes the ones you guys get are identicle, they are a generic model......but they do not have to be. one T78 may have a completely different sized turbine or compressor wheel than another T78 or T88. anyways....
Turbonetics ....they designate their turbos by the actual compressor wheel diameter in milimeters. (this is a far better way..it really gets you in the ballpark) A T-72 is a big turbo....real big! SW just ran 150mph on one with a 75shot.
the T68 is a new one to me....I did not find it in the turbonetics catalog (maybe it is a new one ?????) anyhow that would also be a monster. I think a T66 is about the limit of a truely streetable turbo on these cars. sure you can go bigger....but you really start sacrificing drivability.
as for as you using two turbos that size!?!?!? no way man.....no F'ing way. It would be drag only....and you would need at least 150shot to spool them.
now different sized turbos. this presents a real engineering problem (first of all can you name one manuf. that has ever done this?) I am not aware of any car with different size turbos....sure they have sequential...but they are identicle turbos, hence the name "twin".
The problem is that you get unequal backpressures. A turbo that has more rotating mass (lets go ahead and assume that they have the same turbine wheel for simplicity sake) would take more pressure to spool. So if you had a T72 and a T68 most of the pressure would try to escape through the easier to spool T68....exhaust tries to take the easiest path!! so what would happen is that you would just end up spinning the HELL out of the lighter turbo......the bigger turbo would never get in its efficiency range and the smaller one would be spun way past it.
this is just a bad idea all around.
I could go on....but I think I made my point.