tl;dr- By saying that $\text{A}$ and $\text{B}$ are orthogonal, you've explicitly defined them as being unable to meaningfully interact. This question seems to be if this situation is even possible if we allow a mutually interactive $\text{C}$ between them. The answer is that, yes, $\text{C}$ can exist while $\text{A}$ and $\text{B}$ maintain complete orthogonality.
Understanding an orthogonal reality is impossible by definition
No entity in $\text{A}$ could ever understand, know of, or meaningfully interact with an entity in $\text{B}$, even if an entity in $\text{C}$ attempts to serve as a bridge. If $\text{C}$ attempted to tell If $\text{A}$ or $\text{B}$ about the other, it'd be meaningless gibberish to their ear.
The issue is that minds are themselves physical entities, and cannot have any congruence with a fully orthogonal externality. Therefore, it is impossible for any mind in $\text{A}$ to reason about anything in $\text{B}$.
Example: Orthogonal beings can't understand $1+1=2$
For a concrete example, consider $1+1=2$. Really easy, right? Your brain solves this question in two ways:
1. Memorization: You've seen $1+1=2$ a million times; so much so that, even if you didn't know what it actually meant, you could recite it.
2. 283
Understanding: You know how numbers and addition work, and $1+1=2$ is really easy for your brain to reason out.
It's possible for $\text{C}$ to get $\text{A}$ or $\text{B}$ to memorize $1+1=2$, in the sense that $\text{C}$ can teach them to reproduce symbols that $\text{C}$ would recognize as representing $1+1=2$. But, if that fact's orthogonal to their reality, then they're unable to understand it, even if they've memorized the symbols that $\text{C}$ can interpret as that fact.
If $\text{C}$ could eventually teach $\text{A}$ or $\text{B}$ what $1+1=2$ means, i.e. addition, then they weren't fully orthogonal in the first place, violating the premise of the question.
Note: $\text{C}$ is underdefined
In the question statement, $\text{C}$ is defined as not being fully orthogonal to either $\text{A}$ or $\text{B}$, enabling it to act as a mediator. However, this description could be true in several ways. For example, $\text{C}$ could be a strict superset of $\text{A}$ and $\text{B}$. In this case, $\text{C}$ can fully understand everything in both $\text{A}$ and $\text{B}$.
However, it's possible that $\text{C}$ only shares a single aspect in common with $\text{A}$, and a different, single commonality with $\text{B}$. In this case, while $\text{C}$ isn't fully orthogonal to either $\text{A}$ or $\text{B}$, it would be pretty close.
This isn't a problem for the question because $\text{A}$ and $\text{B}$ will never see each other as more than pure, random, senseless noise, regardless of $\text{C}$'s exact nature. 610