OK. So what do you mean by saying "they transfer mono"? Is it:
1. GarageBand records only left or right channel (waveform display on GB shows 2 waveforms on the track, but one is just straight line)
If this, you may have wrong, faulty or misconnected cable between Portastudio and Mac.
2. GarageBand records both channels, but they are the same (two waves on GB waform display, but plays on-the-center of stereo image)
If this, have you connected your PortaStudio's PHONES out to Mac's input and pressed Portastudio's MONO button?
3. GarageBand records a mono track (only one waveshown on the waveform display on GB's track)
Tell GB to record a stereo track (don't remember how ... been more than 5 years since I tried GarageBand).
Yes you indeed can transfer 1 track (or 2 tracks) a time into GarageBand, but synchronizing the tracks may be horrible (if not impossible) job.But if you succeed you can then edit tracks, add effects, which may (or may not) be worth of trouble.