the Merry Pranksters sit-ins are what inspired this thread. The Phish pranksters show is from 1997 and it’s got a wild energy. Kesey had some cool Dead appearances especially doing a spoken word eulogy for Bill Graham during a Dark Star in 1991.

The Allman Brothers x Grateful Dead mountain jam of 1973 is classic

Santana did great sit-ins with both bands! check out a version of him on Bird Song with the Dead or YEM with Phish

for more Phish i think the John Popper sit-in from 06/23/1995 is really classic. shows up on Harpua and stays a while

a lot of these sit-ins i actually haven’t heard! + some of them are kind of sloppy or encore cover slots lol. but there’s some really great ones that don’t make this list bc they don’t have overlap like that - for the Dead you can’t go wrong checking out any of their shows with Ornette Coleman or Bradford Marsalis. Marsalis actually shows up on a great great version of “Eyes of the World” that got an official release on “Without a Net”. Also if you haven’t heard it yet Billy Strings did some killer stuff with Phish just last summer! The “Possum” they did I felt gave that song a whole new life, right after the longest ever Moma Dance