Multiple Controller Setup?

[claysf]

I'm slo-o-o-wly but surely getting my portable studio set up, but keep running into questions that are over my head.

What I have:

Dell D600 laptop with 1.25 Gigs of RAM
M-Audio Fastrack Pro USB interface(audio in and out; One MIDI In and one Out)
Cubase SX
Garritan Personal Orchestra
1 Roland HP-1 piano in Studio A; a Yamaha Clavinova in Studio B (both are 88 key, excellent playing 'weighted' keyboards with no controll functions other than CC64 sustain pedal.) Both have MIDI In/Out/Thru
A Roland U-20 keyboard/synth with mod wheel and three assignable slider-controllers (essential for playing GPO expressively) with MIDI In/Out/Thru

I want to use both the U-20 AND one of the 'good' keyboards as controllers, simultaneously: the U-20 for its controllers, and the pianos for their playability. How would I hook this up? Do I need some sort of junction box that takes two MIDI inputs and feeds them into my interface? Or, hopefully, is there a 'trick' way to do it with what I already have?

Help greatly appreciated!