With compound turbos, the large turbo should flow about twice as much CFM as the small one. Twins aren't actually "twins". To use two HX35's like you are talking about you would have to feed each one with three cylinders, there would be no compound effect and in that case you would probably be better off getting two smaller turbos as two HX35's would be too big. This type of application is common for gassers, but not diesels. Not saying it wouldn't work, you just probably wouldn't gain anything by doing it.
Nick