You can use fire pads in this situation, but it must say in the pad listing that this application is ok. Allied makes a box that only requires 3" of seperation and no stud between them.
Be very careful on mulitfamily projects. If you use fire pads they must also have a sound transmission rating of 50 or higher and not all of them have it. You could have them do a field test and it only has to be a 42+, but the building has to be done to do this test and that could be an expensive fix.
I can build anything you want if you draw a picture of it on the back of a big enough check.
There's no substitute for hard work....but that doesn't mean I'm going to give up trying to find one.
IAEI / CEI / C10
Certified Electrical Inspector