I love Garageband; it's all I've used for recording music by myself, and it's worked great with my midi-controllers for playing live shows.

The only real issue I have with it is that it does not allow for the layering/coupling of keyboard voices while playing live.

For example, on almost every cheap Casio 88-key I've played, there's a function that allows the player to use two sounds simultaneously (e.g., a harpsichord and organ). This is not to be confused with splitting the keyboard down the middle -- each key played has both tones, to give you a thicker, more unique sound.
Obviously, this doesn't matter for recording, but for practicing and playing live it limits what I can do. For instance, I have one song that uses a grand piano sound coupled with a chorale sound for a full and haunting tone -- live, I am forced to choose between the two.

Does anyone know of any programs in the vein of Garageband that have this layering/coupling feature?
Obviously, I'm asking because I'd rather not buy another keyboard.

crazy irritating, right? it's not possible, without building your own sampler.
apple has a software solution aimed at live playing that's great for layering, called Mainstage
http://www.apple.com/logicpro/mainstage/
recommended. it's only $30.
sequencers like Logic and Digital Performer allow virtual groups of midi tracks, device groups. that's the usual solution, and you'd think GB could that too.
oddly, i don't think Reason, which is usually terrific, allows this. go figure.
good luck.