Using the configuration in my last reply, I am able to independently control both ASI cameras now, no problems connecting. I've taken various test shots, played with gain, etc. I haven't had any clear nights to try guiding yet, but I'm hopeful that since the 120MM-S can be told to capture photos, then that should be enough.
Hardware: ASI183MC main camera, ASI120MM guide camera.
USB3 port on main wired to USB3 port on Pegasus powerbox
USB3 port on guide wired to USB2 port (!) on main, per ZWO recommendation
For the ekos profile, I have only
CCD: ZWO CCD
no entry for guide camera.
Indi seems to want to pick up the guide camera from the main, also somewhere in ZWO or ekos documentation? Not sure.
This profile causes the main camera to use the indi_asi_ccd process, and I think the guide uses it also. I think I was getting some kind of conflicts, maybe at usb bus level, when I had a profile with zwo camera 1 and zwo camera 2, as each one was associated with the indi_asi_single_ccd process..... But all this is speculation, based upon trying a lot of different combinations.