According to IGN on Wednesday, even though the X-Men characters won't be teaming up with the webslinger in "The Amazing Spider-Man 2", there will be a mid-credits scene from "X-Men: Days of Future Past". This seems to demonstrate that 20th Century Fox is crossing over somehow with the competing franchise at some level, even if it means to allow a mere advertisement in their film. To note, this was first discovered in international release of the film and other sources confirmed that this would be seen in the United States version as well.
Very recently, though it's been thought that "never the twain shall meet" between studios, there is an explanation for this exception. Apparently, director Marc Webb was in contract with Fox Searchlight to direct "(500) Days of Summer". Following the first "Spider-Man" film in which negotiations with Sony Pictures had come to a delay, Webb was in conflict with Fox and was allowed to move forward with the Marvel comic book movie. The exception was made only under the condition that Sony Pictures would allow for their "X-Men: Days of Future Past" film tease to be advertised in the mid-credits scene free of charge.
Sony seems to feel that this may benefit them in some fashion by by ensconcing themselves among big franchise of the Marvel Studios family. However, the creative minds of Kevin Feige and other big names in said community are technically separate entities. The consequences of this arrangement may allow comic book fans to be mislead into thinking there may be a X-Men/Spider-Man team-up and if they don't get their wish, this could wind up in frustration for those watching.
Recently, 20th Century Fox did come up with a sizable "X-Men: Days of Future Past" trailer. It now gives a very in-depth synopsis of what happened between the two groups of mutants then and now as well as the common foe they face known as the Sentinels.
"The Amazing-Spider-Man 2" with Andrew Garfield opens in theaters everywhere come May 2.