Answer

Greetings.
This is a general question about a common-denominator derivation, since there too one always learns from two source cases. But precisely because of that, a necessity for both source cases is established between them. When there is no such necessity, meaning when one of the source cases is superfluous, then the rule is stated that two verses that come as one do not teach (because the superfluous verse comes to show that the law was stated only in these two cases and not in other contexts). But when there is a necessity for both source cases, then it is clear why the Torah wrote both of them, so neither is superfluous, and therefore the rule that two verses that come as one do not teach does not apply there. On the contrary, in such a case the two verses are like one, and they teach by means of a paradigm-case derivation.
By the way, this is another reason why, whenever one learns through the common side, one finds a characteristic that exists in one source case but not in the other, and vice versa. Usually this is understood as being done so that we can generalize to the case being learned (to reject objections from each source case individually: when there is an objection from source case A, we say that B proves otherwise, and vice versa). But here you can see that there is another reason for the necessity of both cases: so that the rule that two verses that come as one do not teach will not apply.
All the best,