I guess I don't really understand what you want to do. If you want to layer patches in something other than a Layered Performance, I think you know that you cannot do that on an XP synthesizer. Are you looking for a way around that "limitation"? Can't you construct a User Drum Kit that could give you the drum sounds you want and use it in a (Key Mode) Single Performance? I know what you mean by a "patch drum kit", but I'm very confused as to how two "patch drum kits" could give you much more than you could get using a regular (Rhythm Mode) drum kit. You want two tones played at the same time? Set each tone on adjacent keys in a drum kit and strike both keys simultaneously. If you want "pitched" drum sounds, then you'll just have to sequence a part at a time using the "patch drum kits".

So, on your SC880 a patch can be comprised of 6 parts (equivalent to the 4 tones or "parts" of an XP patch)? What I don't understand is this: "...[in SC880 PATCH mode,] you can layer up to six sounds and still play independent parts with the leftover parts not used in the layer." Is this like an XP Performance Mode that would allow one to set the first six parts to "layer key mode" and the last ten parts to "single key mode"?

Sorry that I may not be fully grasping what you are asking/describing. Good luck, anyway!