I wrote the exact same thing and was also marked wrong. As near as I can guess, I think Duo is expecting both verbs to be in the simple present ("learn") or both verbs to be in the present progressive ("are learning"). I think the engine isn't designed to handle mixing up the two tenses within the same sentence
If there is no continuous aspect, then "You and I learn" should be as correct as "You and I are learning. Most of the time, if I answer with "I learn" or "You learn" Duo says it's correct and adds that another correct answer is "I am learning" or "You are learning." This tells me that both are acceptable because German lacks the continuous aspect. To all of a sudden require a specific answer when both answers are acceptable up to this point, seems like an error on Duo's part. Either there is a continuous aspect or there is not. If there's a rule that a sentence cannot mix "learning" and "are learning" then that needs to be clarified. Even then, technically, this is two independent sentences separated by a semicolon because they are related. It would be just as valid to use a period instead of a semicolon. If in that case, "are learning" and "learn" are interchangeable, is there a reason why German treats the continuous aspect differently in the case where a semicolon is used?
The sentence told me that the correct translation was "You and I are learning, we are learning." But the question was formatted with the list of words and you construct the sentence by tapping the words. But there was only one instance of the word "are" in the list. I physically could not make that sentence work because I didn't have the option.