Hey dave, this is an off the shelf kit that is made by Vecco. ( http://vecco.net ). They are a fairly new company that is now making twin turbo kits. It only took 23 days to get the kit to my door after ordering. I have the stage 3 kit which is capable of 980 hp. It uses T3/T04E hybrid turbos. They are 57 trim with stage 3 wheels and .63 A/R. Check out their site, they have a dyno vid of an otherwise totally stock 5.0 and I highly reccomend them. PM me if you'd like more info or have any questions!
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.
All mustang twin turbo kits use 2 turbos that are the same size. It doesn't take a rocket scientist to tell you what the optimal turbo size for your application is, it just takes a little time and a calculator. I can give you a link if you want to learn to read compressor maps... its not that bad.
<b>A single turbo is better for drag racing.</b> All your king of the hill and pro street turbo mustangs are single, not twin. This is a commonly known thing in the racing community, however the street community always sees different. LAG? Lag is for manual transmissions, automatics with stalls do not have lag at all, they do not have anything besides full powerband all the time. This is why a single turbo is better for drag racing, it can supply more boost. All of you race cars are automatics with stalls, so inheriantly you will not see any twin turbo race cars.. I may have not been clear about this earlier.