I am not sure what the issue is. This is what we have been doing for decades now. If your client does not want to be "inconvenienced" by using multiple remote controls, they can drop some coin for a proper universal remote control system.
If they do not want to drop that coin, they have no room to complain about neither the volume popup message nor the fact that they may have to use 2 remotes otherwise.
"When I eat, it is the food that is scared." - Ron Swanson