"Why not in two days?"
Translation:למה לא מחרתיים?
In my opinion "למה לא ביומיים" and "למה לא בעוד יומיים" should both be accepted as well, but "למה לא ביומיים" has a different meaning than the accepted translation. "למה לא מחרתיים" means something like "Why not in two days from now" - used for example to set a new time for an appointment. "למה לא ביומיים" means something like "Why not in two days in total" - for example to set the amount of time it takes to finish a job (the speaker asks why it cannot be completed in two days).
Thanks. That's super interesting and helpful. Have a lingot. Here's a discussion of this topic in which a person notes that Polish also has a word: https://english.stackexchange.com/questions/51675/is-there-a-one-word-english-term-for-the-day-after-tomorrow