Why cant you use more than one fogger per room?

Why would you want to?

If a statutory label of a toxic product states something in this country (UK) there is a legal requirement to follow those instructions.

You are releasing toxic chemical into a confined space.

If it states the dose then that should be sufficient providing the rest of the instructions have been followed. Doubling up on the dose won't kill whatever the target species is twice as quickly, nor will it kill twice as many, all it will do is double the level of toxicity that you or some other non target will receive.