300.21 requires penetrations through a required fire rated wall to use the proper through connectors,etc.

What section prohibits mounting back to back outlets in the same cavity of a non-fire rated wall? I can't seem to find it.

Thanks!

That is going to be in the building code

Look in ICC 712.3.2 (that's where it is in the ICC based Florida code) Membrane penetrations.
You probably will also get references to the documents for listed fire rated assemblies.
I have never actually had to enforce fire codes but I have taken a lot of CEUs in this area so I am OK if I get corrected.