Sound Forge is not a multi-track recording program like Sonar, which is why you need both. Sound Forge is best used for crafting the final stereo mix that you obtain with Sonar or another multi-track source. I'd highly recommend that you use Sonar for multi-tracking if you already have it.
Having said that, there are probably some tricks you could do to get a quasi-multitrack recording, such as recording your computer's output of Sound Forge to an external mixer and adding in another instrument live while recording to a DAT or some other recorder. Or, if you want to do without stereo then you may be able to combine your stereo tracks to mono in Sound Forge, save the file and reopen it in the left channel and record into the right channel.
_________________________
Jim Eshleman