I found this out myself today and decided to share it. This will allow you to play both modded gamesaves at once. This means you can load up one modded gamesave and another one and have x2 mods. Here is how to do this

What you will need:

-USB
-2 Modded Gamesaves for 2 different accounts
-An Xbox (Duh )

Text Tutorial:

1. Add 2 Different Gamesaves to 2 Different Accounts.
2. Once you have done that, move the gamesaves to your USB and plug it into your Xbox.
3. Load up MW3 and log into the account you want to have the first modded save on.
4. Load up the gamesave in Campaign and wait until your in the game.
5. Once in game, press start and save and quit.
6. Once you are in the campaign lobby, Sign out of your account and log into the 2nd account with the modded save,
7. (Repeat Step 5 again)
8. End the game and go to Special ops.
9. Start any game up and wait until your in it.
10. And there you go. You should have x2 as much as mods now.