Agreed, they just finished uncluster-f**king the franchise with Days of Futures Past. They shouldn't worry about a cross-over or friggin universe building just yet. Or else we'll end up with an X-men variant of TASM2... or Iron man 2. Or Captain America: TFA.

If they really want to, just to peak their slightly inflating eg,, erh curiosity. They could ever so lightly hint at the other's existence in a movie but be very subtle in case one franchise crashes and burns. *cough* Fantastic Four. :(