I can see the Friend Lists of two other users who aren't friends: call them A and B. How can you see all mutual friends between A and B, instead of cross-referencing yourself inefficiently?
This solution fails. I don't understand this other recommendation quoted beneath: what ought be on https://searchisback.com/? It doesn't let me input A or B's names.
Use https://searchisback.com/.
It will generate a search query that looks like this:
https://www.facebook.com/search/ID1/friends/ID2/friends/intersect/
where ID1 and ID2 are the Facebook IDs of the queried users.