Thanks @Guy Lavoie. I'm just finishing a renovation on our house and setting up Insteon and other smart home stuff, and I think I will have that scenario of multiple locations to turn something on, so will refer back to this, or look further into virtual switches.
I'm actually a bit surprised that something like turning off a secondary button isn't built-in to UD...
Btw, unrelated, have you ever done an "all off" scene like in a hotel? Do I just create a scene and add all of the lights, fans, and whatever as responders to that scene, and add secondary Keypadlinc buttons as controllers (like by my front door, garage, etc.)?