Connecting mixer to MacBook Air

Can each mic appear on separate tracks in Garage Band?

no.

from the product description (emphasis added):

Behringer Xenyx X1222USB USB Mixer Features:

• • USB connection for stereo computer recording

you could record on 2 tracks, at most, simultaneously

Can I use just the USB cable to connect the mixer to the laptop?

yes

How do I set up

select it in GB's prefs:

assign the 2 output channels to two tracks:

Because that is what you set on the mixer. Each input on the mixer should have a pan control so that you can set the mic to the left, right or in between. If you want 2 mics on each channel set 2 hard right and 2 hard left.

All GB sees is the 2 L/R stereo channels coming from the mixer. It does not know anything else about the signal.

how to get two separate microphone listings in the GB as the input.

here's a video that explains everything about multi-track recording in about a minute and a half:

https://www.youtube.com/watch?v=-UrRzbLvsqc&list=PLrbREdrV0FcclT--Ay7pZATWGHGRf5M2Q&index=9