Back in the day -like way back in the 1980s- I played at a table where the GM homebrewed a campaign that used AD&D1e, Traveller and Gamma World. IIRC there was some mechanism for dimensional time shifting and distance Universe travel. So the early history era used AD&D and distant future used Traveller. Gama World was a different dimension/plane of existance, that you could access from the other 2. It was a college group and the party of PCs tended to fluctuate in size, so there were times when the party was split up with some playing AD&D1e, some Traveller or some GW, all at the same table.
Very ambitious and seriously crazy for sure, but the GM Michael was one of the cleverest and most skilled I've had the pleasure to sit down at a table with. So against all odds, it worked.