If the voice and synth are hard panned left and right it should be quite easy. Change the balance so you're hard to the side of the voice (effectively muting the synth). Then bounce out by clinking the bounce button on your stereo out, but not before changing its format to mono.
Alternately you could also convert the file to split stereo (its been so long since I've done this I'm not sure, but that may be all you need to do) then set the balance to dead center.
And mute or delete the synth track, of course.....
On the bottom of the track you are wanting to play only one side, hold down the tiny icon that turns the track to mono or stereo. you should see a L as well as an R, One is for left side only, the other for right. just select the one you wish to hear.