'Duo viris studet', I'd have thought, as 'studeo' is one of those verbs that takes the dative.
However, as I pointed out above, the original sentence should still be translatable as 'Duo! The men study!'
Considering that this is the Duolingo Latin course, 'Duo' should also be the vocative of 'Duo'...
The first time on here a language course where it's not meaning the owl
Since there is no article in Latin, duo viri may mean "two men" or "the two men".
This is very confusing if you also learn other languages on here, because Duo inserts himself in so many sentences lol