I understand that the X-Men and the Avengers are both based on Marvel comics series and they seem to often share adventures in the comics (for example Wolverine and the Avengers appear together both in this picture from the Marvel website). Marvel founder Stan Lee also tends to appear in both film series.
But I haven't seen any of them run into each other. Aren't they living on the same planet? Do they ever meet? If not, why not?