For singing in a group and contemporaneously rather than sequentially (oops, swallowed a dictionary!) - a friend of mine gets 1/3 of the room singing "Rock-a-my soul on the bosom of Abraham" - another third on on "He's got the whole world in his hands" - and the last third are on "When the Saints go Marching In" - and it works!

Of course, Gilbert and Sullivan operettas are renowned for their "double choruses" where half the cast is on one song and the other half on something different.

And yes, Pachelbel's Canon, Little Robyn - I think there have been other threads on how many songs are based on that chord sequence (as well as the wonderful disgruntled cello player video!)

And finally, I have heard some folk tack 2 unrelated songs together, just because they can see that in a busy session they're only going to get one turn tonight - is that fair play? Hmm, well tunes players do it all the time - sets of 3 or 4 tunes that go on for 20 minutes!