
In your current setup you'll probably have to use the keyboards' Local On\Off feature. But if you have a onboard sequencer on the keys that you're relying on, then the best method is determined by you as far as which machine will be designated the "master". The master device will always use midi out only. The recieving or slave device will use midi in. Subsequent additional midi instruments can be "daisy chained" by using midi thru starting from the first slaved device in the chain\network to the next devices' midi in/thru an so on. Also, you'll have to understand your midi tracks/midi channels so that you'll be able to set 'em up properly and do work-a-rounds with 'em...like stacking instruments using the same midi number but on different channels. Take the advice and read as much as possible. But from your question, what is the purpose of the mpc being hooked up in the first place if all you wanted was to play keys? Is it that you want drums to play along also? If your answer is yes, then follow the above layout and you should be good.
MPC 4000
SSL Fusion
Roland JV1080
Ensoniq ASR10
Pioneer XDJ RX2
Korg Triton Rack
Kurzweil K2500RS
MiniMoog Voyager
Yamaha Montage 7
Yamaha Motif ES Rack
UA Apollo X8 Interface
Tascam DM4800/DA3000