I don't know what software you're using, but with Cubase (even the older versions) you can easily assign each midi track to a different midi output device.

On my system (with only one soundcard) I can choose between the onboard synth or 'midi out' I see no reason why it wouldn't work with 2 soundcards.

I agree with Jon that if you can get access to some sort of external synth or module, that would probably be the easiest route.

If you'd like a copy of Cubase, PM me for advice.

Ed