I was just thinking the other day that I would like to read one of these historical adventure loyalty/betrayal/honour/etc. type novels, but with two female leads, but I have never come across one.

I have recced this to regshoe before, but: have you read Code Name Verity? It really has a lot of that kind of vibe, but between two women. Well, maybe not much honour, but definitely loyalty and potential betrayal. It's set in World War II.