I would say no based on the exact wording. If two expert witness have different arguments to get to a conclusion, then it may very well be that the argument to get the conclusion is weak. It would absolutely be a fallacy if instead of...
the argument that the sky is blue is weak.
you were to have written
the sky is not blue.
And this is because the nature of a fallacy. A fallacy is an argument, at least one explicit premise leads to a faulty conclusion. Obviously if two experts have differing claims as to why the sky is blue, then to conclude the sky isn't blue would be specious. But, if one defines a "weak argument" as argument that fails to persuade, then to have two experts who cannot persuade each other seems to confirm that both arguments are somehow weak. In fact, the less agreement there is among experts about the why, the more evidence accumulates that no argument is strong.
Remember, as written, the claim of person C has nothing to do with the color of the sky, but rather the strength of the arguments. If we were to reformulate from C's perspective, to get to C's conclusion, it would be more along the lines of:
P1. Expert A's argument does not convince expert B.
P2. Expert B's argument does not convince expert A.
C. The argument to get to the conclusion shared by A and B is weak.
In fact, I would say not only is C not engaging in fallacy, but that C is making the start of a compelling argument.