I only hear sound through one of my headphones when monitoring in Garageband
When you set up a new audio track, select Mono track and channel 1 as the input source (or channel 2 if you have a mic/instrument plugged into the 2nd channel port on the Saffire) to create a Mono track.
Why can't I get any sound through my headphones?
The headphone outputs are 5/6 and 7/8. Make sure that you have audio routed to those outputs in Saffire control (you should also hear the same thing out of line outs 5/6 and 7/8)