First, make sure your original tracks are at 0db so you aren't adding any unnecessary gain or noise to those files.
Then open up your sequence settings (command + 0) and go to the audio output tab. From there you can set how many outputs you want and if you want them stereo or dual mono.
Then go back to your timeline, right click the green enable audio button all the way to the left of each audio track and set each track to the output you want.
Then when you export your movie with current settings it will export out a multi-channel quicktime with each audio track assigned to the outputs you want. You can bring it back into to FCP to make sure it has all the audio.
And if the client can open an OMF you can export an OMF which will keep all your settings for your mix (if you want you can turn that off) and the file is ready to be opened in ProTools and such for further mixing if necessary.