"Oui, vous deux devez."
Pfff I hate these kind of sentences in French. It's doesn't feel natural. It could be used, but in very particular cases. Most of the time we would specify what those two people must do, even if we repeat what the previous person said with a substitute.
For example : "Oui, vous deux devez le faire." = "Yes, you two must do it."
In a French point of view, it would be a lot better.
The correct solution is "Yes, you two must.". What does this mean in English?
It's the same in English as in French (as written above). We would usually say what the two must do.
'yes, you two must do it' or 'yes, you two must clean the floor' or whatever. There are not too many times when we would not specify. And those are during casual conversation where both parties would understand what it was they were talking about.