You'd need new manifolds, dumps, airfilters, piping maybe FMIC.
The stock GTR manifolds doesnt fit on the gtst's (i think).
Basically you'd be spending more $$ than you would if you had a single.
Though it would be pretty cool to see a RB25DETT (I think its been done, but im not sure)
I wanted to do this but.. in the end costs too much