X-Men: Gamma Quest Trilogy by Greg Cox

X-Men and The Avengers: Lost and Found:

The X-Men -- mutant protectors of a world that fears and hates them. The Avengers -- Earth's mightiest heroes, the greatest super-team ever assembled. When the Scarlet Witch of the Avengers and Rogue of the X-Men all disappear under mysterious circumstances, each team's search leads them to more questions than answers.
When the X-Men appear to attack a high-security government installation, the Avengers must track down the mutants, and learn the truth behind the abductions before it's too late



X-Men and the Avengers: Search and Rescue:

The X-Men--mutant protectors of a world that fears and hates them. The Avengers--Earth's mightiest heroes, the greatest super-team ever assembled. Now they must put aside their differences and join forces against the Leader, the gamma-powered evil genius who has already abducted one Avenger and two X-Men as part of his latest plan--with the fate of the entire globe at stake!



X-Men and the Avengers: Friend or Foe?:

The greatest super-teams ever assembled: the X-Men -- mutant protectors of a world that fears and hates them -- and the Avengers -- Earth's mightiest heroes. Now they have joined forces with the incredible Hulk against the deadly leader, having traced him to his base on the moon...