VG30DE - Saab's light pressure turbo 3.0L V-6 which came out 2 or 3 years ago just uses one bank of cylinders to spin the turbo; the other bank do not enter the exhaust side of the turbo at all. As long as it's designed with that in mind in the first place, it can work just fine. Both banks don't have to enter the turbo.
With regard to dual vs. single - most any of the high HP, for example, Supras (I'm talking 500-900 rwhp) get rid of the factory twin turbo set up and use one big-arse turbo. Most twin set ups use one small turbo that spools quickly and builds boost at low rpm, and a second larger turbo which is designed to deliver it's boost/flows at higher rpm. That way the whole rpm range is covered. They work great, are very streetable -- not uncommon to see peak torque at 1800 rpm and peak HP at 5500 rpm, for example. But it takes a rocket scientist to select the proper turbos, and work the computer controls to get everything working together properly. For simplicity, I'd probably go with a single on a V-8. Check out this month's Pop. Hot Rodding to see Mark Stielow's new 65 Chevelle with twin turbo-intercooled small block. A small fortune in mandrel bent stainless steel 90 degree ells.