Garageband multitrack recording issue

Hi - I'm fairly new to macs and Garageband (GB).

I am trying to record with two microphones simultaneously through a (MOTU M6) multichannel interface. I have record enabled two tracks in GB and have asigned the correct input channel to each (1 and 3). I have also record-enabled each track (button goes black - but only when the track is selected) and can record each separately. However, when I try to record from both at the same time GB only records from the track that is highlighted.

What am I doing wrong that stops simultaneous recording?