It sounds like a second Soundcard would solve your problem (to me). It's true, a few years ago it could get very tricky, but with modern pnp, it should be OK - I run 2 pci pnp cards _and_ an ISA non-pnp, and the all work together. Of course, running in sync is another matter. And whatever you do, don't ever get two card of the same vzariety unless they are specifically designed to work together.
I think that having one soundcard with a multi-client driver might also solve the problem, but you are uinlikely to find such for a built-in jobby.
If you have two SCards then you should be able to tell the software in the softsynth which one to use, and like wise in cakewalk. If it's anything like pro-audio, each track should have an output option. You can mix the relative levels of the two using the windows level mixer thingy (if necessary).
Good luck!
matt