I know this has been covered in another thread - maybe search "matrix" to find it. Here's the basics using Cool Edit, since Cool edit will allow you to load both sources into the same multi-track workspace (Sound Forge would only let you do this by placing one source in the clipboard - much harder to work with). 1st, cut one source into tracks with something like CDWave. 2nd, load the "untracked" source into Cool Edit's multi-track view. 3rd, load the other source into Cool Edit, lining it up track by track. Last, do a mixdown.