You want 2 turbo's on the RB20?
Firstly if it's a stock RB20, you are better off with a swap to an RB25det turbo. The twins are going add useless complexity.
If assuming you have a 'built' (read forged pistons etc..) RB20 with larger cams and so on. You could custom make some exhaust manifolds for a twin setup. Even twin factory sized GTR turbos will kill a standard RB20 with the airflow they can deliver, and the fact that you will need to rev the tits off the engine to get it into the power band.
What I am basically saying is......If you want more power throw the RB20 away and get a 'real' motor it will cost you less in the long run.