Apparently I've been involved in a debate in the Duolingo forums for about a year on whether when two possessors in a possessive construction possess the same shared/common object, whether the object should be in the singular or in the plural. At least in this particular construction.
People there have been discussing more than one aspect of the Korean and English versions of the same sentence, including previous versions of the sentence which may no longer be shown.
But this one point seems to have people defending both views:
It is two teachers' birthday today.
It is two teachers' birthdays today.
Depending on how my brain is working, both can sound right or one sounds right and the other sounds wrong. In the thread linked I defended the position that both should be regarded as acceptable.
Right now I have convinced my brain that only the first is correct due to agreement between the Singular pronoun It, singular verb is, and singular object of possession birthday. Is this correct?
(The thread can be confusing as to which person is debating which point of grammar in some comments and replies.)