multi-track recording

I am using
an Alesis Multimix 4 USB FX mixer
windows 10
Audacity 2.3.2

I am using two channels on my mixer - one microphone plugged into each channel.
I would like to record each of these channels on a separate track within audacity.
Under preferences I have selected devices then under recording I have selected “line (USB audio codec)” Then under channel I have selected “2 (stereo)”

When I push record it only records on one channel.
What I am doing wrong?
Thanks for your help.

The Alesis Multimix 4 USB FX mixer provides 2 channels (stereo) over USB.

The MultiMix 4 USB FX enables bidirectional > stereo > playback, > so you can record the stereo Main output > into your computer and listen back using the mixer as an interface from software to headphones and monitoring loudspeakers.

(emphasis mine).

Check in the Windows Sound control panel that the Alesis Multimix is set for 2 channel stereo recording (Windows will often default to 1 channel mono).
You will then be able to record 2 separate channels, by setting Audacity to record 2 channels stereo. Anything that you want in the first channel you need to “pan left” on the mixer, and anything you want to record in the second channel needs to be “panned right”. Audacity will then record as a stereo track, which you can then “split” into two mono tracks (see: Splitting and Joining Stereo Tracks - Audacity Manual)