I also don't know exactly what you mean. You can control several sounds via different midi channels and switch between them. But if you want to cross over from one to another sound while playing it then you'll probably need a synth which provides the ability to create multi layer sounds plus a controller which allows you to balance between both layers (something like a cross over controller). Many synths have single and multi sounds but there are less which came with a sampler. Kurzweil built a synth with an optional sampler on board. There are keyboard and rack versions.
I don't know if you can do the same with pure samplers like AKAI or ENSONIQUE. Maybe there's somebody who knows more???

Maybe it's a midi controller what you're searching for. There are programmable midi foot pedals which allows you to control special midi functions. They're special made for life acts...

I hope I could help a little bit. Good luck!

------------------
Greetings from Frankfurt (Germany),
Sheriff ;-)

[This message has been edited by Sheriff (edited 10-04-2005).]