CIV 5 and MAFIA 2 combined - works?

Like I said, the key you activate on steam (and the game) would not be revoked because they are completely different entities. you are fine.

Yeah, I think he's basically asking "Can GMG and/or Steam track keys if they want to, and would there potentially be any problems if someone had activated 2 keys originating from this promotion on their account?"

I doubt there would be any problems, but that's mainly an uninformed guess.

In general terms, I'd actually be interested to know who can find out what about keys and on which accounts they end up on etc. Can a retailer find out where the keys they sold go? Can a publisher? Can Steam find out which retailer keys came from and when they were issued etc?