Ok, so just to confirm, we’re now working on the utility room light, which is controlled by two switches. One of those switches is in the single-gang box, the other is the switch on the LEFT in the 2-gang box. Good so far?
So what this appears to be is a Line and Load in separate boxes. It looks like power is being fed at the 2-gang box. What you will need to do to confirm is in the 2-gang box, remove the black conductor attached to the black screw so it’s no longer attached to the switch. USING A METER, test between that black conductor and the white bundle. You should get a constant 120VAC (or something close). Just to be thorough, test with the switches in various positions. That should not matter, as you should have a constant 120VAC.
If that is the case, then your power (Line) is being fed from the black bundle in the 2-gang box. The Load to the light(s) is the black of the 2-wire in the single gang box.
So the Inovelli will go in the 2-gang box. Use the following diagram. (And while the switches are out, run the screws down into the switch bodies. Thx)
Tagging @harjms for a 2nd look . . .