Gary Morrison wrote:

The planetessimals would be colliding with one another...

In saying that, I'm assuming that planetary accresion is a greadual
process, at least on a time scale of hundreds of thousands of years,
giving plenty of time for the planetessimals in the two orbits to
collide with one another. Perhaps the answer is that planetary
accresion happened a lot faster than I suspect.

Or perhaps the orbit of Orpheus was a comet-like, extremely non-circular
one, thereby giving it a very long orbital period thereby making the
opportinities for collision comparatively rare - long enough for planets
to form. If that were the case, though, I'd go back to my earlier
concern, that such an impact would seem highly likely to throw the
resulting Earth into a very non-circular orbit - in essence, sort of the
average of the two planets' orbits.