"Both of them have a birthday."
Translation:יש לשניהם יום הולדת.
Although Pumbush already said his, it might not be clear enough so I will say it as well so learners are not mislead: one of the "correct" answers given is: "לשניהן יש יום הולדת".
There is no such word as לשניהן! It should have been: לשתיהן.
The Hebrew sentence above is wrong. is should be Shteihen or Shneihem. There isn't such word Sneihen.
So you are right.