Joint ownership of one apple? Not grammatically incorrect but not a phrase I would expect to hear very often.
We shouldn't be picky with the nouns that Duolingo gives us. For all we know, they might be in a "biggest apple" competition. We can replace apple with anything: dog, house, or car.
This sentence is meant for Poles and people learning Polish in New York City. :)
The Big Apple... so we're talking about New York here? :D our New York :D