In 10.2v how do I record two tracks at same time?

I am trying to record both the vocals and guitar at the same time. I am using an M-audio interface so I have a separate channel for audio and guitar. I thought I could just highlight both tracks and record but that doesn't seem to work. But I want them as separate tracks so I can adjust them separately later. I realize there will be some bleed of the guitar into the microphone but that is OK. I just can't seem to get Garageband to let me record two tracks at the same time. Is there some other way to do it?