Xbox – Can you use Split Screen multiplayer in Modern Warfare 3 for LAN Party

The system tries to match you up with similarly "ranked" players with the same available map packs. If it can't do that, it will try to match you up with players with a subset of the same available map packs. All of this is automatic and seamless. I have two 360's and for a while one of them didn't have the map pack installed. I didn't notice a lack of games. My friends don't have the map pack, they join my party and we play as usual. No special hoops to jump through.

I have, on many occasions, had both my US account and my Canadian account on the same Xbox.

While it may seem strange that I would have both, after living in Canada for a number of years I became frustrated with the restrictions (I couldn't buy gamer points in Canada on my American account). As a result I now have both.

This has worked seamlessly on both my American and (later) my Canadian Xbox.

While I can't be 100% sure about Brazil+US, I do know that Canada and US are separate regions and they do function as you would expect.