Yeah, but the thing with Scarlet Witch and Quicksilver is that they're both heavily connected to the X-Men and The Avengers. Thus the contract they worked out goes such that Fox can use them, but not make any mention their of affiliation with The Avengers, while Marvel can use them, but not make any mention of their mutant background, their connection to the X-Men, or Magneto (their father).
Thaid said, it's obviously not impossible that Marvel/Disney and Sony could make a deal to do a crossover, or possibly even merge their franchises; it would most likely be very profitable. I just don't think it's remotely likely. Even though there's profit in it for them both, I can't imagine they could ever negotiate a deal that both parties would be happy about. Besides, to make things fit together, they'd might have to reboot or retcon Spider-Man
again, and that wouldn't be very popular or smart..