I took it that he was commenting that the mercenary nature of the two enemies would lead them to unite the moment it would be to their benefit to do.
I thought the same thing Kuanyin.
Also Barnabas is there for a reason and that is to save David from total possesion by Quentin's ghost.
It's obvious that Barnabas knows that Quentin fate was sealed at a young age & he more than likely knows that it was probably a tragic one at that, and also that it's possibly connected to Chris Jennings in some strange way.
Given these little known facts, Barnabas knows that probably sometime soon in the future that Quentin is going to be needing his help in some way, although he doesn't know what kind of help as of yet. And of course, Barnabas realizes that in order to help David, he needs to help Quentin first.