Recording two tracks simultaneously

Using GarageBand (10.0.2) and a Universal Audio Apollo Twin Duo, I want to record one acoustic guitar using two GarageBand tracks and getting it all in one take.

This means I will play the track one time and record it on two tracks at the same time.

I intend to pan one track full left and the other full right and manipulate the EQ, f/x and mix on each one independently and after the fact.

I did it before a couple of years ago so I know it can be done.

I have completely forgotten how I did it.

I've tried a few things already and none so far have worked.

I've gone into the UA mixing console and set up the two inputs and balanced them out.

The need for balancing comes from two discrete audio sources being used at the same time, those being a vocal mic to capture natural sound of the guitar and the room ambience and also an acoustic amp direct line out for the other one to capture the sound of the electronics of the guitar.

It is pretty common for pro studios to do this with acoustic guitars, having two audio sources and then blending them in the final mix-down.

I have the amp line out connected to one UA input and the mic connected to the other.

Within GarageBand I selected an input track that uses both inputs 1 and 2 and that didn't work.

I only got the mic.

The other side of the stereo input was not recorded despite the UA mixer showing both signals bouncing.

I tried selecting two tracks in GarageBand with one using input 1 and the other using input 2 but when I go to record only the selected one gets recorded.

I know there's a way to do this because I've done it before.

I just can't remember how.

I do remember I fought with it for a long time before I found the solution.

I'm hoping to avoid that frustration this time.

Any help given will be appreciated.

Thanks