It's been known for a while now that Quicksilve and Scarlet Witch are appearing in the upcoming Avengers sequel. Joss Whedon confirmed this. To shock of all nerds; Brian Singer have recently tweeded that Quicksilver will also appear on upcoming X-Men: Days of Future Past, played by Evan Peters.
Theory 1)
X-Men franchise and Avengers/MCU franchise shouldn't have anything to do with each other, since they are currently owned by two different studios. Quicksilver and Scarlet Witch are however two few grey-area-characters, which both studios have right to use, because they are both mutants and long time iconic Avengers members in comics.
After they were announced for MCU, many have been speculating that how will their origins be told, since they can't be mutants. Will they be presented as inhumans, or will it be that they were born with their powers, without that word "mutant" is being used. The theory that popped into my head, after hearing that they (or at least Quicksilver) will appear on both franchises: Avengers 2 comes out less than a year after the X-Men:DOFP, so could it be that, in order avoid to invent new origins to characters, Marvel has some short of deal with Fox that thurtful character origins will be told in the X-Men movies, and then Marvel could easily take them to Avengers team without wasting time to introduction. I'M NOT TALKING ABOUT SHARED UNIVERSES HERE! I'm just saying that studios could do some kind of cooperation in order to get the most out the character. This would also make sense when you think that in X-Men franchise the charakter devolopment would most likely focus on their relantioship with their father: Magneto, while in the MCU they can't approach this matter at all due the character rights.
Theory 2)
It could be that, due to either Fox's complain to Marvel about swooping the characters, or the Marvel's realisation that Avengers/MCU is getting too constricted with characters, that studios have made an agreement to spilt the twins: Qiucksilver will go with X-Men while Scarlet Witch shall be part of the MCU. This would make sense when you think that Quicksilver has always been more connected with the daddy issues, while Scarlet Witch has been more close to Avengers from two of those (she even married an avenger; Vison). Also Scarlet Witch presence is not that inportanto for X-Men film-universe; what Phoenix did at Alcatraz in the end of X3 has sometimes been refered as the M-day of the film-universe. In MCU, Scarlet Witch could be lead up to Doctor Strange, who has been rumored to appear on phase 3.
So what do you think? You have your own theories? Leave comments :)