Just wondering... Have you considered just doing a TwinScroll turbo instead? Twinscrolls have the advantage of being able to spool quickly and make pretty decent power mid range without having the hassles of twin turbos.
I would venture to say that the twinscroll route would probably be a little better all around, but thats just me.
If your going to do 2 parallel turbos though, you'll probably have to jump down to a TD04 from the wrx, twin VF40's probably wouldnt even get you power until 3500 RPM's or better running on just 2 cylinders.
If I pass you on the right, I'm flipping you off.