Was there an agreement between 20th Century Fox and Marvel Studios for Wanda and Pietro Maximoff?
First, let's discuss the live action character rights for these two characters. In the comics Scarlet Witch and Quicksilver are mutants, so technically they fall under the movie rights agreement Marvel made with Fox who own movie rights to all-things X-Men and mutant. However, the history of these two are deeply involved with both the X-Men and some of the earliest Avengers comics dating back to the 60s. So there's a bit of a gray area with them, which is why you see both studios using them.
In X-MEN: DAYS OF FUTURE PAST, Fox introduced actor Evan Peters as their version of Pietro, a.k.a. Quicksilver, but no sign of his twin sister. In AVENGERS: AGE OF ULTRON, Marvel introduced both the twins Wanda, a.k.a. Scarlet Witch, and Quicksilver (played by Aaron Taylor-Johnson) but he did not make it and died at the end of the film. In Age of Ultron, they were not the children of Magneto and they were the result of Baron Strucker's human experiments using the scepter which housed the Mind Stone.
So now Fox is using Quicksilver and Peters' role in X-MEN: APOCALYPSE seems to be much bigger than in X-MEN: DAYS OF FUTURE PAST. Including a mention that Magneto is his father in the latest trailer. And Marvel Studios has Elizabeth Olsen as Scarlet Witch in their Marvel Cinematic Universe and her presence is expanding in CAPTAIN AMERICA: CIVIL WAR as a part of #TeamCap.
So the question needs to be asked again... With Fox having Quicksilver in their X-Men movies right now and Marvel having Scarlet Witch in the MCU, was a deal brokered between the two studios for each to only use one of the twins in their respective movie universes?
Nobody has brought up this observation and this question needed to be asked.