It takes a whole week before they meet up again, and Elsa still doesn't know why she asked that question. She doesn't blame Anna for not answering, either.
It was stupid and foolish, but... some small part of her had hoped Anna would say yes. Would agree to intertwine their lives again, this time on purpose because... they were friends. To Elsa, at least.
When they meet up, it's because Elsa can't get to sleep with someone ringing her buzzer, and she... doesn't really want to when she hears Anna's voice from the intercom.