A couple of months ago Joss Whedon hinted the inclusion of 2 new characters for Avengers 2.  He said they are twins, brother and sister.  Then just last week, Whedon confirmed that he is including Quicksilver (Pietro Maximoff) and Scarlet Witch (Wanda maximoff) on the Avengers roster.  Whedon mentioned that the 2 are first members of the Avengers so they are allowed to use them in Marvel Cinematic Universe without conflicts with FOX who holds the X-Men franchise.  Everybody rejoiced for this news not until minutes ago when Bryan Singer of X-Men Days of Future Past announced a new cast and you won't believe who he is.

Before he was an #Avenger, he was just a REALLY fast kid. Thrilled to say #EvanPeters is joining #XMen #DaysOfFuturePast as #Quicksilver.
— Bryan Singer (@BryanSinger) May 23, 2013

So Quicksilver is now confirmed for 2 superhero franchise. There is no clear explanation how will this new development can affect Walt Disney and 20th Century Fox but if we will speculate, there is only 3 options: a) they both use it and make them stand on their own without continuity, b) Disney and FOX will soon have an agreement and connect FOX's Marvel Cinematic Universe to Disney's or c) One of them will give up the character all together.

If it is the later, Joss is still on a draft phase of the script and may end up giving up Quicksilver (and probably also Wanda). I would really like if Disney and FOX will come to an agreement and merge their respective universe.

As of now we don't have a clear answer so we will just wait and see.

X-Men Days of Future Past (2014) is from Marvel Studios, to be distributed by 20th Century Fox through Warner Bros. while Avengers 2 (2015) is also from Marvel Studios to be distributed by Walt Disney.