Viva Audacity!
I like being able to make a project with two stereo sources ---
--- timing, leveling and enveloping each source in relation to the other
--- saving the result to a matrix.wav
--- loading the matrix.wav back into the project and solo-ing to check the result for chattiness or overs
--- making alterations to the envelopes to generate a better matrix2.wav (without having to kill the project)
It's free, and really all you need..