I’m not familiar with that interface. Audacity may require you to record in stereo (2 channels, one track) but it each channel of the stereo track should contain the signal from one microphone only. Then do “Split Stereo to Mono” from the Audio Track Drop-down menu.
Sound mixers almost always have, in addition to all the fancy controls for a single microphone, a PAN control.
This lets you push the microphone sound to the left or right or any combination. You plug one microphone into CHANNEL 1 and one into CHANNEL 2. Pan 1 all the way to the left and pan 2 all the way to the right. Split the stereo show to two mono shows using Audacity, as Billw58 above.
If you don’t have those controls or something like them, then this exercise may not work for you. Audacity will not accept two different interfaces at the same time without serious additional software and fiddling.
I don’t think the PreSonus Audiobox 96 has controls like this, although it might. If you record in stereo, it might naturally send one microphone to the left and one to the right. consult your instructions. Split as above.
I believe you should be able to do this with no problems. (What happened when you tried?) As Bill indicated, the two channels will come through as a single stereo track, so you may want to do the “Split Stereo to Mono” thing that he mentioned.
The PreSonus Audiobox 9 does NOT have a PAN control, so this option will not be available to you, but I believe the interface provides the individual channels for you. (Some mixers provide only the mix). For USB recording, I believe you just need to set the Mic1 and Mic2 level controls. I don’t believe the Main, Mixer, and Headphone controls are needed at this point.
According to the manual, “Your AudioBox USB96 is a class-compliant Core Audio device in macOS. No
driver installation is necessary. Simply connect it to your computer.” However, I did notice a “Universal Control” software download for MAC available at the Presonus web site here: https://www.presonus.com/products/AudioBox-USB-96/downloads. So if you can’t get it working out of the box, that is something extra that you could look into.
For Audacity, make sure the Presonus Box is connected and turned on before starting Audacity; otherwise, you will have to do Transport > Rescan Audio Devices. Click on Audio Setup, then select the Audiobox as your Recording Device. Recording Channels will be 2.
Audacity will remember these devices AS LONG AS THEY ARE ALWAYS PRESENT when you start up Audacity. Otherwise, Audacity will revert to the defaults, and you will have to use the Audio Setup button again.
Also note, you may need to grant permission for Audacity to use the microphone.