The two principal characters exist in a complete story because they simultaneously compliment each other at the same time they both oppose one another. Looking at the very top of the Dramatica Table of Story Elements--where you find Genre level story points like the Main Character and Influence Character Domains--one can see this dynamic connection.
Arranged in a diagonal configuration, both MC and IC Throughlines will be found either in Situation and Fixed Attitude OR Psychology and Activitiy. The former will be both States (Situation and Fixed Attitude), the latter will be both Processes (Psychology and Activities). This is where the complimentary nature comes from. The opposing viewpoint comes from one being External and the other Internal.
As revealed in the article Two Sides of the Same Coin, this reality of story often results in the cornball line, "You and I are both alike." The MC or the IC recognizes some kind of shared problem and they can't help but deliver a ham-fisted line. There are, however, sophisticated ways of getting the same information across. One such example exists in Tom McCarthy's 2003 film The Station Agent.
Main Character Fin (Peter Dinklage) is a dwarf--a dwarf with a huge chip on his shoulder. Spending a life dealing with laughter and ridicule at the expense of your appearance tends to make one a wee bit bitter. Fin is angry about being a dwarf and he wants out. In an attempt to distract himself, Fin buries his head in the past--preferring the predictability of trains and their set paths over the often cruel and unpredictable reality of human nature.
Influence Character Olivia Harris (Patricia Clarkson) suffers from a different, albeit no less painful, fate--the tragedy of losing a child. Fixating on the memory of her son and desperately wanting her old life back, Olivia finds herself stuck. She wants her old life back and it won't come back to her.
And here is where the similarity between the two characters exists.
Olivia is stuck with her grief; Fin can't not be a dwarf. They're both stuck (one internal, one external) and they both can't let go. She won't give it up; Fin can't give it up. Olivia is dealing with the same issues as Fin, but from the point of view of attitude. There is nothing external keeping her stuck mentally that way, it is all internal. Contrast that with Fin who is all about being stuck externally.
Regardless of internal or external, the unchanging nature of their individual problems brings them together and sets them against one another. It is why this story has to be about Fin and Olivia and not about Fin and Steve or Olivia and Jason.
Finding a Subtle Form of Expression
You don't need to have the "You and I are both alike" scene in your own story. As long as you find that commonality of issues between the Main and Influence character--as The Station Agent did--the audience will understand where you are coming from and appreciate the narrative you create for them. Sophistication in the alignment of conflict alleviates the need for obvious dialogue and insures a story with purpose and meaning.