Probably the biggest MK fan here (I believe his work with Slash eclipses Guns 'n' Roses, and I was there the first time) but I'm scratching my head on this one. To me, "vocal harmonies" are two or three people, singing complementary lines (not, usually, the same line) simultaneously. There's little of that on any of the Slash records. Listen to the intro to "I Would Have Waited Forever" by Yes, or "Because" by the Beatles for an example.
I know Alter Bridge uses a lot of self-backing vocals where you would hear Myles sing with himself like in AB's Calm the Fire. Sometimes it works. Sometimes it gets annoying and some people do not like the self-backing vocal trick, especially when Mark Tremonti is a good enough backup vocalist to fill in the blanks without it being processed, I guess?
Aside from that, the only examples I can think of AB really utilizing vocal harmonies is Come to Life in the chorus, Words Darker Than Their Wings, and maybe Before Tomorrow Comes, especially live (even though people do not want to hear it live anymore, but that's a different topic for another day).
As for Myles with Slash and the Conspirators, I think, in general, can't recall a specific song, but he and, the bassist, Todd Kerns meshes really vocally well and can play off and complement each others' voice well.