Thanks Orac. I begin to see the sort of mechanism that would make preearth's scenario possible.

Just to check my thinking thus far; would it be right that the fact that the two bodies are orbiting would result in a gentler impact than if they were on a direct collision course? Also, their counter-rotation (provided the speeds matched) would help to prevent them from ripping each other to bits and flinging the debris into space?

There never was nothing.