It should tell you how to do that in the manual. All recorders have a diffurent way of bounceing so i could not tell you how to. go look in your manual and see if it tell's you.
I dont think this model supports ping ponging tracks if I remember correctly.I've only seen one other recorder that wouldnt bounce internally and that was the bottom of the line tascam 4-track cassette a few years back..I forget the model.I just read the manual for the X-12 and it doesnt even mention Bouncing tracks(Ping Ponging).....doesnt look good.Here's the manual if you dont have a hard copy of it.It's the third one down on the list: http://www.fostex.com/Support/manuals.html