Found a workaround that lets you use the keys:
TLDR:
I used lines 3/4 as (alt) monitor outputs and then ran the signal to a headphone amp. You can then control the volume with the built-in keyboard shortcuts when you are in alt (yellow) mode (
instructions on how to get to alt mode with the keyboard below).
How to:
1) Make sure lines 3/4 are set as
alt monitor outputs. Do this by setting "alt count" to 1.
2) On the twin, connect a TRS
mono-to-stereo splitter to outputs 3/4 to turn the mono outputs to stereo.
3) Connect the stereo end of the splitter to a headphone amp. I used
the Behringer HA-400. The cabling required for your amp may vary.
4) To switch to the alt outputs with the keyboard, you need the
UA Companion app (costs $20 but comes with a trial). This lets you
set keyboard shortcuts for just about anything on the Apollo except the headphone volume. (The developer for the app confirmed to me that there's no way to do this with software).
5) You can now control the alt monitor volume with
Cmd+f10/f11 (on an
M1 Mac, this is
Cmd+fn+f10/f11). You can also toggle to the regular monitor outs and adjust the volume with the same keys.
*you can do this
without the headphone amp, but the sound quality wasn't good enough for me personally
*Sadly, you won't get volume control through the Apollo if you want to use line 3/4 simultaneously with the regular monitor outs because you'd have to disable the alt monitor mode.
