Thanks for the reply. In the time since I started this thread, I decided to scrap the idea of a dream sequence as a plot device for getting the characters to cooperate. I was trying to negotiate an iron triangle of avoiding cliches, keeping it plausible, and staying true to the "personalities" that I have envisioned for the characters. It was too much of a pain in the ass, so I changed the storyline a little.
The first time they meet, they try to kill each other and are forced to stop fighting before one can score a killing blow. The second time, they are forced to cooperate because of a common enemy. Once that danger has passed, the two conclude that they're "even" and part ways. At this point, they each have separate encounters with the entity I was referring to, and only after that do they meet for a third time. It kind of avoids creating the problem I was talking about in the beginning.
The supernatural agent in the story will be more subtle. It will appear in the form of a person who has always been familiar to the character. I was thinking of something along the lines of Krishna in the Bhagavad-Gita. Arjuna doesn't really know that his chariot driver is a god for most of the story but still calls on this person for his advice.
I can give you a more detailed explanation, but I'm getting ready to head out so I'll have to post it in an update.