In the situation where there are clearly A B and C available as options; saying either
"A and B are the stronger of the arguments."
"A and B are the strongest of the arguments."
Both unambiguously state that A and B are stronger than C (or "the strongest arguments"). Personally I'd go with the first, but it's a matter of style. Also I think the second flows better as:
"A and B are the strongest arguments."
Purely as a matter of style, as all three options in this case mean the same thing. Similarly, "Of the two apples, this is the larger" and "Of the two apples, this is the largest" really have little difference between them.
Bear in mind that neither of your sentences heavily stress the equal strength of A and B, just that they are both stronger than C. (It has an air or implication that they are both on roughly the same level with respect to C, as in "out of Ireland, Brazil and Morocco, Brazil and Morocco are the hotter Countries" but doesn't preclude that, say, B may be the stronger of A and B.)
As a side note, contrary to some answers here, there is nothing wrong with using a superlative to refer to a plural.
"Grandma, these are the best cookies I have ever tasted"
is fine, regardless of whether or not there is a better or worse cookie in the batch or whether they are all equally good.