is there a way to use 2 sounds from garage band at once using midi keyboard

http://store.apple.com/1-800-MY-APPLE/WebObjects/AppleStore.woa/6244002/wo/NU22K lvYKl3P3noXuvM7UfSwAr1/1.0.21.1.0.8.25.7.11.0.3

If that doesn't work (I vaguely remember the Apple Store using cookies), just search for "Jam Pack" at the store.

GB can only take input on a single MIDI track at a time.

You'd have to find an instrument that contained both sets of samples. For example, Jam pack 1 has a "Piano and Strings" instrument

~~HangTime V1.9.9.9.8 [Will Compute for Food] B-)>

Hmmm, I read that as: He wanted to play both parts live.

But if I read that wrong, then yeah, what Christoph said, you can create any number of tracks (up to 64 MIDI, or as many as your machine will handle) and have them all playing at once while you play live on one track.

~~HangTime V1.9.9.9.8 [Will Compute for Food] B-)>

You can duplicate the regions of one track to another with the second instrument.