I'm looking for articles that help explain the two sides of the same coin concept, but can't find anything.

I don't know where there are specific articles on the "You and I are alike" dichotomy, but the concept is simple:

IN THE BEGINNING...

In the back story (for a Change Main Character**) or at the beginning of the story (for a Steadfast Man Character**), there comes a point where the Main Character must choose a path to take because of some PERSONAL inequity or imbalance introduced by an event of some sort. The Main Character then goes down that path attempting to resolve the personal problem. The Influence Character represents the path not chosen -- the path that is intimately tied to that original choice consciously or unconsciously made by the Main Character at the point when and where the original inequity was addressed.

WE ARE THE SAME...

The part of the argument that ties the two perspectives together, those of the Main Character and Influence Character, is the point of origin -- the event that introduced the original inequity. They both have some relationship to the core inequity that is both the source of personal conflict for the Main Character, but also is the source of the Main Character's drive. This is what gives them a basis in similarity.

WE'RE NOTHING ALIKE...

The part of the argument where the Main Character and Influence Character diverge is the path taken/chosen to address the original inequity. The Main Character represents the path taken. The Influence Character represents the path NOT taken by the Main Character and is the alternative to the Main Character's path. That is WHY the Influence Character cannot be ignored by the Main Character. The Influence Character represents a legitimate means to addressing the original inequity. However, legitimate does not mean it is the "right" (effective) means to address the "problem."

This divergence in paths/approaches to resolving the Main Character's inequity creates a tug-of-war between the two characters. There is no way for the Main Character to know if it is on the right path toward resolving it's personal problems, or if the Influence Character's path is the better of the two.

WE'RE JUST ALIKE, YOU AND I...

So, with the Main Character representing one path and the Influence Character representing the alternative path, a storytelling convention has emerged where the Main Character and Influence Character have a conversation that establishes this relationship. It often goes something like this:

IC: We're the same.
MC: No, we're not the same. You [insert an example of the different path]...
IC: True, but you [insert an example of the shared attention to the inequity], just like me.

... or an interchange that effectively communicates the same information.

In short order, the author has informed the audience about:

• The Main Character's position on addressing the Main Character's personal problem
• The Influence Character's alternative position on addressing the Main Character's personal problem
• How the Main Character and Influence Character are similar in their approaches
• How the Main Character and Influence Character are dissimilar in their approaches

STRUCTURALLY SPEAKING...

In the storyform, the most visible expression of the Main Character/Influence Character approach divergence is seen at the Class level of the structure. One character searches for the solution externally (Situation or Activities), while the other uses an internal approach to resolving the inequity (Fixed Attitude or Manipulation/Psychology). That explains the "not alike" part of the argument.

The part that explains the similarity of their approaches relates to the axis of their dynamic (diagonal) pair relationship in the structure. Both characters will have throughlines in EITHER domains that explore processes (i.e. Activity and Manipulation) OR domains that explore the state of things (i.e. Situation and Fixed Attitude).

In this way the two have a basis in common ground (state or process) as well as a divergence in approach (internal or external).

THE GRAND ARGUMENT STORY

A grand argument story does not begin until all four throughlines are present. [NOTE: This is not the same as how the story is presented to the audience through storyweaving. The AUDIENCE may not be aware of the presence of all four throughlines at the beginning of the work, but each of the four throughlines must be evident BEFORE the first act turn, and preferably much earlier than that point in the story.] A key part of the Main Character's purpose in the story is to explore the path it has taken in its attempt to resolve its personal issues. That exploration is unlikely to occur without the irritating effects on the Main Character's complacency (if any) by the Influence Character exploration (or embodiment) of the path NOT taken by the Main Character.

The inciting event sets into motion the collision (and cohesion) of the four throughlines that form the underlying basis of the story and the drive towards its resolution (or non-resolution).

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** As a general rule, the Main Character's personal inequity is established in the back story for Change Main Characters and at the beginning of the story for Steadfast Main Characters, but there are many exceptions to this rule, especially in stories that don't end well for the Main Character (Judgment: Bad).  

More specifically, how can Support be my MC's Symptom if Disbelief is his Problem? I don't under the context of Support here.

It makes a bit of difference if your MC is a change character or a steadfast character, but let's assume he's a change character for a moment.

In a story like this, the MC with THINK his problem comes from support, and he THINKS the solution to his problem is to oppose.  What he doesn't realize until late in the story is that he is truly driven by disbelief and only if he has faith will he have the possibility of resolving his personal issues.

For example, George hates it when his step-father tries to be supportive and responds by begin opposing every effort to engage him.  George thinks that as long as he plays naysayer he can keep himself from being hurt further.  As a START character he holds back when a bit of effort on his part could smooth out conflict, and only after time and a lot of resistance does George eventually get to the point where he can recognize that it is his own fundamental disbelief that anyone would want to be his father is at the heart of his fear of rejection and his dream that his biological father will return.  At a critical moment in his personal struggle, George chooses to have faith that there can be someone who cares about him enough to become the father he never dared to think he deserved...