Well, American English is still English. But it should allow "you all" or simply "you" without requiring "y'all". There was likely something else wrong with your sentence. Sometimes Duolingo seems only to catch one mistake and it generally corrects it to whatever correct answer it thought was closest to your answer, not necessarily the best or most common answer.
Or without introducing the word sharing and taking eat out;
Jullie eten één boterham.
Jullie zijn één boterham aan het eten. (But that is stil a bit ambiguous)
Jullie eten samen één boterham.
Jullie zijn samen één boterham aan het eten.
You are eating one sandwich together.
Sharing is indeed the clearest but changes the sentence a lot.