The use of multiple main characters within a text tend to play off of each other. Their characteristics tend to either mirror or oppose the other--in a sense, showing two sides to one distinct personality (seen when a doppelganger is present).
For example, if one were to look at the two main characters in Mary Shelleys' Frankenstein, one would see that Victor and his creature act as opposites to each other. In much of Gothic literature, people (characters) are understood to be both good and evil (the balance remaining ambiguous, or unknown). Each time Victor makes a decision about an action, the consequences are enacted by the creature. Therefore, in this circumstance, the two main characters play off of each other (one's good actions bring about the bad actions of another).
In another example, one can look at L. Frank Baum's The Wonderful Wizard of Oz. In this text, it takes Dorothy's travels with the Scarecrow, the Lion, and the Tin Man to complete her journey through Oz and make it back to Kansas. One could look at each character playing upon the other in order to create one "character" in the end. All of the main characters are necessary in order to get Dorothy back to Kansas. Their similar needs insure their success, while their differences bring to the table the different characteristics needed to succeed.
Last, but not least, some main characters are simply created in order to show the conflict which arises between two different people. For example, in John Steinbeck's Of Mice and Men, the characters of George and Lennie conflict on a daily basis. Their relationship shows how they conflict with each other and with society at large. The story is one of two friends, in search of The American Dream, struggling to find a place for themselves in the world. These two main characters show their love for their dreams (similarity) while looking for the dream from two distinctly different personalities (differences).
Therefore, typical similarities seen in two main characters are the similarities in personality, ideology, behavior, need, and actions. Differences between two characters can follow suit; two main characters can be different in personality, ideology, behavior, need, and actions.