There are MANY different combos that you could use. I notice that you are Mitchellized, but you trying to build your own twins?
I have heard of combos like... . Hx35 with a 22exhaust housing, and a Garret, or an HX40 with a 16 and a H2E, the combos are endless. I think If I were trying to build a twin turbo system, and had plenty of money to play with, I would try a Pius(NOT PIERS)-set up H2E with a 16 housing, and a HX60 with a 18 housing. Another combo that I think would be cool is a Piers HX 40 with a 16 housing, and a H2E with a 18 housing.
These are just ideas though.
Andrew