If only Gerard and Daphne were in the minds of the writers in 1968. XD 1840 was just another storyline added after everything that happened in 1970PT, as we all know.
I think that you were looking for a different type of answer, so here you go. Personally, if I were to believe in ghosts haunting houses...they would only exist to affect the present day residents according to how they died and whatnot. They wouldn't have any effect on the "plans" of other ghosts.
On another note, since Gerard was "destined" to destroy Collinwood in 1970 (or so I remember), his ghost wouldn't even start to affect the household until the year in question. Quentin's and Beth's ghosts, on the other hand, were not "released" until they spoke on the old phone. Then, their spirits were put to rest before 1970, therefore quelling any possible "meeting" of ghosts.