I say this all over the web over and over again:
The key piece of advice regarding forum administration is that forums are delicate creatures that feed on eyeballs and voices. Users expect a good forum to be a buzzing hub of activity, so they need to be above a critical mass to survive. Most people are reluctant to post in old threads because they want a conversation, active threads are stale if the last post was older than a few days ago.
The correct time to split a forum depends on your threshold for noise, if new and/or important threads are quickly being pushed off the first page (within a few hours), the first thing that should be done is to ignore it for as long as is possible!
If you fork at the peak of a popularity spike then you run the risk of starving all forums when the new traffic dries up, this usually causes all your old users to gravitate around the most popular one while the others are neglected, which turns new users off as they now have a smaller chance of selecting the active one for support, then don't bother registering when they see how dead it is. This causes a slow death as more users leave than join, the Internet is littered with the remnants of once-popular megaforums.
If and when you do split, do very carefully, in response to long-term organic growth and with the sole intention of filtering noise.
It's much harder to merge forums than it is to split them, everyone has to agree that the community isn't as big as it was, or that new forum was a failed experiment. Objectivity is obviously key, but people (myself included) rarely swallow their pride before its too late. If the top threads are sat idle for days and you can see weeks into the past on the first page then it's time for that merge.
One day I'm going to write a spider to prove or disprove these hypotheses, but in the meantime please take my unsupported opinions as wisdom!