"Slate Rock and Gravel" from The Flintstones and the Jetsons #1 (DC, 1997) by Mike Carlin, Glen Hanson and Mike DeCarlo
Nah, they don't actually team up through some kind of time warp or suspended animation. Each family had its own tales, usually linked thematically. This is the only issue that found its way into my collection, but I've heard good things about the other 18 issues, and I'm especially interested in issue 8 where the Great Gazoo apparently turns Fred and Barney into Wilma and Betty! I wonder what the Jetsons are up to in their story given that "theme"!