It also depends on how many cylinders are in your "bank". Are we talking 1/2 of a V8 here? Does one I6 count as a bank? I also assume that you're talking about equal total flow between setups? (ie. 2x 25lb/min turbos versus 1x 50lb/min turbo)

There is incresed exhaust flow efficiency by having fewer runners per turbo. This is the idea behind twin-scroll technology. That is one thing going for a twin setup over one single-scroll turbo. The other idea I've heard tossed around is that twin smaller turbos have lower total inertia than a big single, which helps them spool faster (assuming both setups support equal flow). This argument was countered with accusations that the inertia difference only applied to older turbos, and that newer turbos are designed with a lot less inertia per flow.