Absolutely right.I use both, mainly to stretch out my dollar as oyster shell isn't cheap (I buy 100% flaked shells, not the "pelleted" kind) and because some of my birds are less enthusiastic about oyster shell.
Eggshells taken strictly from your flock don't provide enough extra calcium in that there's no way to extract 100% of the calcium that went into them out of them. If you feed strictly layer feed just eggshell may be enough to meet your layers' needs due to the additional 4% calcium in the feed, but if you feed grower or all flock, you will probably need some oyster shell or other form of calcium as well to meet their needs.
I give mine eggshells, oyster shell and Cuttlefish bone.