Correct, a sound card with only a single stereo output cannot drive two separate stereo setups. I suppose, though, that if you can deal with mono sound, you could pan one source all left and one all right, and use a stereo splitter on the output to drive two setups. With either the front/rear split or the left/right split, though, I think you might wind up with some degree of crossover between the signals, as sound hardware is generally expecting all of the channels to have somewhat similar signals on them.