Apple Logic Pro /LogicExpress Discussion

I'm rather new to Logic and I have not played with MainStage, just the multitrack/mixer portion of Logic.  If I want to have use the Yamaha Piano sound, I create a new software instrument and load it into that track.  If I want a Wurlitzer, I build another track. 

I have a live gig approaching and I'm realizing I can't be switching from track to track (i.e. use the mouse) during an individual song to get different sounds.  In the past w/o Logic and just using the keyboards built-in patches, I'd layer the patches so they could quickly be turned on/off with a single button on the keyboard itself.  The other option was to split the keyboard and say put an Alto Sax on the top octave that I might not be using for that song anyways.  I could of course do that again, but I love the patch library and effects in Logic.

In Logic, what's the best way to layer sounds or split the keyboard?  Should I be doing this sort of thing in MainStage?  Any other suggestions?  (I have a Roland A90 controller)

Thanks,
Bryan Lucas