I have Battlefield 3 on Xbox 360 and I have to put in Disc 2 (Single Player) and then select Multiplayer and then insert Disc 1 (Multiplayer disc) to play. Shouldn't I just be able to put in the Multiplayer Disc and be able to get online?
You should be able to use each disc independently. Either disc will ask if you want to install the HD texture pack, which is recommended. I played the co op & multiplayer myself before, I started the campaign. If you still can't start the game, you may want to see if you can swap discs back at the store.