I believe only a court could definitively answer your question.
The way that item is made out is not consistent with generally accepted banking practices. A check can be payable three different ways: jointly, alternatively, or ambiguously.
I believe your item is ambiguous and, as such, could legally be deemed payable alternatively, meaning that only one of the parties would need to endorse.
However, I think an argument could also be made that it was payable jointly. (I think it's a weak argument, but it could be made.)