Xbox – Modern Warfare 3 Multiplayer with Guest

Its like using the same account on two xbox. You can't do it.

Now there are obvious reasons why Microsoft doesn't want you to be able to use the same account more than once (which are compounded by DLC), but the bottom line is: you can only be logged into the same account in one location at a time.

Now you might want to consider the new Family Pack, which is coming soon. Or streaming your Netflix to a PC.

The short answer though, is: No

You can be in a party while playing split-screen, but unless the second player has an Xbox Live account, they won't be able to participate in any of the voice chat options that Xbox Live offers. You can plug a headset into their controller, but they won't hear the party/game chat audio, and their mic won't function.

If the options to join a party aren't on the same buttons in-game when you're playing split screen, you can always use the Xbox Guide button to access the party menu and start/join a party from that interface.

Note also that you have to have a second controller plugged in/turned on in order for the "Press A to Join" message to appear in most games (MW3 included). There's nothing in MW3 or Xbox Live that prevents you from doing both at the same time.