"Two apartments" was accepted, but whether you believe me or not, it is a misconception (Konglish, if you will) that 아파트 means "apartment" (or "flat" or "unit"). Many Korean people make this mistake and even make offical signs that say, e.g., "ABC Apartment," when they actually mean to name the entire building, not an individual home inside that building. (In my example, "ABC Apartments" would be correct, with the plural 's' added.) 아파트 refers to the entire building, so the correct translation here is "two apartment buildings" or "two apartment houses".
Well actually... What I heard is that it's because before apartment buildings were built, individual rooms in large, old houses were rented out and that meant that the kitchen and bathroom were communal. Everyone renting a room had to share these communal areas. Apartment buildings were designed to allow for cheap, independent accomodation (with your OWN personal kitchen and bathroom), all the while being "stuck" with many other people. So "apart" even though all together.
DuoLingo just confused me:
Growing up Korean-native, with a Korean-speaking family, it was my understanding that 아파트 means "apartment". This would result in a literal translation of "two apartments". Why is the word "buildings" there? Is it because "Two apartments" is a stand-alone statement without much description?
Please fill me in on this, I've seen similar translations where they edit the overall sentence to only follow the generic meaning of a phrase and I've never understood the point.