FCP isn’t really good for audio-mixes.
Easy way would be to ask the sound dude/girl that did your mixing for these 2 sets of tracks directly out of his/her/their sound suite. Would be pretty easy. Then you should do the “Copy of sequence – delete original audio – re-import” thing mentioned in the end.
If, on the other hand – you have done the mixing in FCP only, you’ll have to mute relevant tracks and create two mixdowns through “export audio”.
Haven’t tried this route myself, but if you remember to “nest sequence-mixdown audio” before you export it sorta should work.
Then make a copy of your sequence, take away your original audio from the copy and re-import the two stereo-pairs.
Finally you’ll have to route the different tracks to the appropriate tracks on the tape given that you have the right deck/soundcard solution.
I’ve never done mor than two tracks in DV-CAM.
Best of luck
Gunleik