With today's ball bearing turbochargers, going with twin setups or God forbid, a seqential setup, is almost like taking a step backwards. Back in the day of the twin turbo Supra's and RX-7's ball bearing turbos were very expensive and almost non existant, so it made sense to go with sequential setups to reduce lag and still make good top end power. Now you can get large ball bearing turbochargers that outspool the smaller turbos that were around back in the day. Take a look at the spool characteristics of a large ball bearing turbo like HKS's GT series.
Also, most vehicles that did come with twin or sequential had a much larger volume of exhaust flow than our engines have. I dont know if you know how much exhaust an RX-7 puts out, but it is massive. Take a look at some of the single turbo conversions done on steet driven FD RX-7's and pay attention to the turbine side housing size. Massive. These cars can spool these monsters easily. Now to use twins on a Forenza/Reno, you are going to have to go with much smaller size turbines due to each only having two cylinders to spool each turbine. You are going to end up with much more backpressure than if you went with a larger single turbo. If you wanted to make some nice power I would reccomend a HKS GT-RS turbo, these make great top end power and have quick spool characteristics.
On another note, two turbos, custom twin manifold, intercooler piping, and all this will add up to more $$$. Rebuilds will be much more expensive if ever needed as well. Oh and do we even have space for two turbos and all the piping?
On to the discussion of deisel turbos, this is one we actually tried just because we had one laying around. We put a turbo from a Marine Corps AAV on a Nissan Silvia with a 2.0 liter SR20DET. Let me tell you, it made outstanding power......after 6000 RPM! Most deisel turbochargers are the old bushing style and the impellers are quite heavy and take a while to get up and moving. The car had nothing at low RPM. Another turbo besides the GT-RS is a Garret T04E. I had one on another Silvia with the 1.8 liter CA18DET. I had a .50 AR compressor and a .50 turbine housing with a ball bearing center section that I used for drifting and the response was amazing. Very little turbo lag and was great all around. Probably my favorite of any turbocharger I have personally used. I was running around 350 horsepower (sorry didnt dyno it) at 19 psi.
Anyway, sorry about the rant, just Greg's 2 cents.