that is pretty much it. You can have one in ERG and the other in Level or power reading only mode. But, you will need a way to connect to both apps and depending on what trainer you have and the connection you use, you may need more.
Eg. I have a Kickr '18 and it allows 3 simultaneous Bluetooth connections so I can connect via Bluetooth to SYSTM at the same time I connect to Zwift (as a power source only, if I want SYSTM to control my trainer) AND, theoretically, Rouvy or some other app as well, though I’ve never done that.
Most trainer’s Bluetooth, afaik, only allow for 1 connection via Bluetooth and the first one to get it, keeps it. So, you could connect to one app via Bluetooth, and another app via Ant+ since most trainers broadcast both at the same time.
Does this help?
Edit: Also, did you ever get your FB drama concluded Sir Joe?