The Maximoff twins, children of Magneto, have a long history in both X-Men and Avengers comics.
They've both been villains and heroes and back and forth several times over. Now they're once again juggling two worlds, but this time it's two film worlds.
Today came news that Whedon and Marvel Studios have almost certainly found their Wanda Maximoff, Scarlet Witch, in Elizabeth Olsen. Rumored for the role for the last couple of months, it seems Samuel L. Jackson let slip the confirmation. Likewise, Kickass star Aaron Johnson has spoken at length about playing Quicksilver for Marvel Studios, something not yet 100% confirmed by either party.
Of course, we know that Quicksilver also appears in X-Men: Days of Future Past from Fox, by way of Evan Peters. That film has already completed principal photography.
So how does that happen? They're a peculiar case, as they're mutants and the children of Magneto, and would thus seem to fall into the Fox-licensed X-Men franchise, yet they're both closely associated with the Avengers, and were in fact some of the earliest recruits to the team in the comic books. Basically, if Fox wanted to use them, they could, or if Marvel Studios wanted to, they could as well. And it looks like they both want to.
How is this all going to work? Details are still very fuzzy, but here are our five best guesses, focused primarily on Pietro, as he's the only confirmed one appearing in both.