They only dropped the Avatar title in the US. Everywhere else, it was Avatar: The Legend of Korra. Might wind up with the same thing here. James Cameron also might only have the rights to the Avatar name for movies and not television.

I was also hoping the show would have dual avatars as the twins, each with two elements.

I really liked this too, but someone talked me out of this idea for a fic, actually. The reasoning being that this is the Earth Avatar Cycle, so it would be almost canon breaking for one to not have earthbending. They wouldn't go through the stages of learning each element, it would kinda throw out the "four elements" synergy and fights and lore. They reminded me that the trope of twins being very close and "two parts of a whole" actually irritates many RL twins (hence's Ty Lee's character).

So my take is twins that each have four elements, but inclinations to two each, as well as yin yang kind of personality conflicts.