There's a good explanation of triangulation here - divide and conquer is often but not always triangulation.
My mother had a nice sideline in telling me or my dad that we were annoyed at each other's behaviour, as she knew we were closer to each other than we were to her. Sadly for her, we started comparing notes when I was about 13.
dogdays you've just reminded me of our wedding photo album debacle. We initially planned to get married just the two of us until my mother guilt tripped me into inviting her but it was still tiny (8 people), I wore a second hand bridesmaid's dress, DH wore a kilt for a laugh - it was not a big stylish affair. My mother asked if she could hire a professional photographer, we said no. OK, she says, but I want copies of all the photos so that I can do a big album (for her, not for us). OK. FIL (also narcissist, but the more normal pain the arse variety, not actually personality disordered) turns up with a massive SLR camera and shoots loudly throughout the ceremony till I tell him to sod off.
Approx 20 minutes after we come back home from 2 day honeymoon, my mother phones up and asks after the negatives from FiL. DH phones FiL, tries and fails on several attempts to get him to send them ("no I'm not going to send them, that won't be necessary, but wait till you see what I am making for you!" DH tries to explain that it's not us that wants the negatives, or indeed any of the photos, to no avail).
About 2 months later we receive from FiL a photo album which he put together himself. Although it's meant for us, we didn't actually want it, and my mother is still asking after the negatives to do her own album, so we give her FiL's album. She empties FiL's album, bins it, and puts the photos into her own album.
(We took our little digital camera to the wedding and DF and DBiL took some photos with it. I put them in an album. That's actually the only photos we have of the day!)