Interestingly, my paperback copy of the Rodman Philbrick novel is actually entitled The Mighty rather than its earlier original title of Freak the Mighty. The 1998 film version (starring Sharon Stone as Gwen, Harry Dean Stanton as Grim, Gena Rowlands as Gram, and Kieran Culkin as Kevin/Freak) was also retitled The Mighty. Philbrick's sequel was titled Max the Mighty (1998). My guess is that the word "freak" was considered too politically incorrect to continue its useage. However, I think that is ridiculous since the main character, Kevin, apparently didn't mind being called Freak; and the two-some of Max and Kevin seemed to thrive on their combined Freak the Mighty nom de pleur.
So, what would I call the book? I think the original title (and the two boys' self-given nickname) is one of the book's strengths. I wouldn't change it. But, since you asked, how about "Two Heads are Better Than One."
Freak the Mighty is the story of the relationship between two boys, Maxwell and Kevin. Maxwell is a slow learner who had been in special classes. He has low self-esteem and feels dumb. His father is in prison for murdering his mother. Freak is a small fellow who has a large head and tiny body. He has never seen the world from above until Max begins to carry him around on his shoulders. He is highly intelligent.
Freak introduces Max to the idea of Camelot and the Knights. He calls his mother the Fair Gwen in reference top her beauty and the Queen of Camelot. One of the ideas for another title could be derived from the boy’s use of this motif in play.
Some ideas could be:
Max and Freak the Knights of Camelot; Adventurers in Arms; or even Max and Freak the Greats.
Think about the dynamics of the story and the boy's personalities and traits.