I just stumbled onto this info the other day.
PolyPhontics
www.bestsoftwaredesign.com
It's $60 and it doesn't even have velocity layering! In my opinion, without that, it's nearly worthless.
I read all of it's documentation and haven't played with the demo yet, but I doubt it can combine multiple SF2's. It's really basic.
Note: the nskit6 parts that were supposed to work by themselves, they don't work in my (latest) metro G4. The nskit5 SF2's do though.
Also I tried using vienna on Virtual PC, but that didn't work. It needs to talk to the Sound Blaster Card.
-M