Can I have 2 XBOX Live accounts from different countries on the same 360?
Solution 1:
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.