It's pretty straightforward [see the attached screenshot]..
- First, select the correct input for each track In the Smart Controls panel [shortcut "B"]
- Record-arm both tracks
- Turn monitoring on for both tracks [the little orange buttons next to the record buttons]; be sure to monitor in headphones [to avoid leakage and feedback from the vocal mic]
- If the interface supports it you can set the level of each incoming audio signal in the Smart Controls panel with the Record Level slider[s] -- if it doesn't then you'd set the incoming [record] level with the appropriate level control[s] on the interface itself
- Set the monitoring [playback] level of each track with the volume slider[s] in the track header[s]
- Hit record [shortcut "R"]
I've also linked this thread to a basic GarageBand course [above] that might be of some help.