Translation:The two of them will have left each other already.
I think the trouble is the future perfect , there really isn't much cause to use it in everyday conversational English. The ATM machine likes to tell me," We will have processed your card details in a few moments" and even that sounds ridiculously convoluted. I suppose I will have learned someting by the end of the exercise.
I used "They two..." which was rejected. I agree with jlco, but if Duo is going to continue with the ridiculous Italian idioms, which are the last thing anyone needs to learn while developing a working facility with the language, it should at least be facile itself with what constitutes proper English translations. I wonder if anybody in charge ever reads this stuff...
"They will already have left each other" is how ANY English person would say this. And I dont care what any grammar book says, Grammar books dont talk. What is the benefit of having to guess an answer that no one would ever say because it is so corrupted.
Yes, because of the si in the sentence. "Those two will have already left" could mean they left the party together. It's not the same meaning as they left each other. Often Duo allows you to leave out the "each other" when the meaning is clear without it. "Si hanno già incontrati - They have already met." But in this case it needs to be put in for clarity.
As I've pointed out elsewhere: dialects are a thing. 'Them two' is commonly used in some parts of the US. It should also be noted that DL specifically requests that you read the comments before posting, as your issue may have been addressed/brought up already, such as in this case.
The phrase is not very useful but it is grammatically correct. Here is a weird example:
"Per quando la verità nel loro matrimonio è esposta, loro due si saranno lasciati già."
"For when the truth in their marriage is exposed, the two of them will have left each other already."