Xbox – Entering Xbox Live redeem codes online (rather than on Xbox 360 console)

Yes, he can either bring his profile with him on a Memory card or USB stick (assuming you both have updated your consoles). I do this regularly when going to a friends house.

He could also recover his account onto your Xbox, but that is a bit more of a hassle.

Achievements and other online stats are synced in either case (as long as you are connected to the internet), but saved game files won't be transferred if he recovers his account (or if he doesn't bring them on his memory card/usb stick).

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.