"Comment vous soutenir tous les deux ?"
I think this is a French idiom whereby the "pourrais-je" or possibly "pourrait-on" is suppressed, to make the question more gnomic/generalised. I.e. we infer "Comment pourrais-je vous soutenir tous les deux?" or "Comment pourrait-on vous soutenir tous les deux?" It's also similar to how in English we say "I know how to swim." but in French "Je sais nager." is sufficient.
The fact that "soutenir" is not conjugated lets us know "vous" is not the subject of the sentence, but the object, and that naturally leads us to infer that "tous les deux" refers to "vous", not anyone else.