Greyhawk and Faerun on the same planet?

[Alzrius]

I guess while it is possible to mash both worlds together. Using Spelljammer to link them adds to the feel of exploration in the game and I find makes things more fun. Plus you can make them Crash on Dark Sun on the way there...If you like to watch your PC's squirm for a bit.

You'd have a hard time having them crash there, as the crystal sphere surrounding Athas is impenetrable, and thus no spelljammer can reach it. Planar travel is the only way to get there, and that's difficult at best.