Also, you might want to make do your mixing in 32 bit and dither later.
options, settings, multitrack, mixdowns - set to 32 bit
Just look out for any clipping when you add the two sources together. You may have to back down the one that's set at 0 (because they add together). Pick the loudest spot, do the mixdown and see if it clips. You can keep the same balance by lowering each source by the same amount.
Whatcha mixing?