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.
Best Answer
Follow this simple technique:
Find Profile ID of
B- say it is Profile_BGo to FB profile of
A- say URL for which isfacebook.com/profile_AAppend URL with following text
?and=Profile_B. The final URL looks likefacebook.com/profile_A?and=Profile_BIt would be finally redirected as
https://www.facebook.com/friendship/Profile_A/Profile_BYou should be good with enough information.