iPod Can this be done?

I've got all my music on my iMac.

I stream music to my TV from iTunes on my iMac (works GREAT).

I have an airport express in a 3rd room attached to a stereo. I stream music from my iTunes on my iMac there, too. (works GREAT).

Does anyone know of a way I can stream DIFFERENT streams to each remote source? Let's say metal in the living room, and piano music in the 3rd room....all using my one iMac as the main source.

I've looked, but can't find a way to open two copies of iTunes at once on my iMac. I can use a laptop as one streaming source, but I want to see if I can source 2 different streams from one computer.

Does anyone have any suggestions?

THANKS!!