I think the Moosemoulds bullet, and the Davidson ones have only one diameter, and if something similar had two, the engraved one would surely be too short. I'd sooner see even those started with a cylindrical ball starter and plunger. Some of the late scheutzen ones had a bayonet fitting to hold them on the muzzle, and compound levers like a little reloading press.
To work the two-diameter principle with only the ramrod or a hand ball seater, I think you would need the portions of both diameters to be somewhat longer. But there is no reason why they shouldn't be.