The version I heard usually involved a lot of complicated mechanics like a voting system.
Instead of a complicated way to split a religion in two just make a way for a new religion to be linked to an old one and allow for easier conversion. This makes it more player controlled. There'd probably have to be an anti abuse rule that the new religion must be based on the old one.
You assume making religions "linked" is actually easier to code. I'm not saying that it is or isn't, been too long since I looked at that database structure, but chances are it's not actually reducing the implementation effort even if it is conceptually "simpler"
