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I don't think I've ever come across what you're describing. But something that may be useful that is similar is:
https://www.lorenzostanco.com/lab/steam/friends

If you add both you, and the friend you want to compare with, make sure you un-tick the 'load only games at least 2 of you own' filter. It will load the two libraries. It always sorts with the shared games at the top, so if you scroll down you will be able to see the games that only one of you own underneath, with a little strip of colour differentiating who owns it, and who doesn't.

I remember seeing that before as well, a dropdown menu where you could choose what you wanted to see (so, both own, not both own, etc.) and it's gone now... but not sure if it was a Steam thing and they changed it, if it was an Augmented Steam and they changed it or?

Oh how i want this. I don't remember it being a steam feature, most likely some extension. I don't know why did't they already do it, with the other filter option already implemented this seems like an obvious simple thing to do, and very useful feature. It'd be in the logic of their development as a storefront ultimately, one of the great means of exploring and discovering new games to buy.