Actually you can do that on a portastudio 02, what you do is, you take the tracks that you have, mix them down to a tape deck in stereo (if your recording in stereo), then you put the tape that you mixed down on into the porta 02 (or whatever you have). After you've done that pan tracks 1, and 2, hard left and right. Vvvwhalla you have 2 extra tracks. Technically you can do this over and over again but the sound quality drops a little everytime.
Oh yea, and if you do this turn down the volume farther than you ussually would on the instruments panned to the center and close to the center.