Yeah don't go with writing a letter, unfortunately many people would think that's weird. Realize that in the end, if things do work out, you will have to make a choice between the two and that could be problematic. You need to be honest with them though and explain everything that happened. If you want to talk to a friend of theirs it may be easier, but the best way to solve this is to go straight to them. The one girl seems to have no interest whatsoever, so if I were you I'd focus on explaining your situation to the other girl first. Be sure to apologize for the miscommunication and if she felt uncomfortable as a result of that. But I agree with what some of the other people wrote here, you should probably take advantage of the fact you don't see these girls as often in school anymore and try to move on. Good luck with whatever you decide to do!