Conventional wisdom says that worms will double their biomass every three months. We usually start with about half the worms our bins are designed to hold. A well-managed bin will hold more worms that they are designed for.

I would wait until the bin is about 6 months old.

I divided my bin the first time I harvested. There was a layer of tightly clumped undecomposed bedding at the bottom of the bin. I had already shredded cardboard for new bedding. I bedded a new bin, and tossed the clumped bedding in another to let it break down some more. The clumped bedding had enough worms in it to populate the bin. It was not long before I had two productive bins.

Divide however you want. You could just grab a handful of bedding from the top of the existing bin, and toss it in the new bin, worms and all. It should work just fine.

My worm bins are rubbermaid style bins. Splitting means a lot less worms in a large space. I put a chunk of styrofoam across the width to make a smaller living area for the worms. They were easy to find, to be able to mate. It wasn't long before I could remove the styrofoam and let them use the whole bin.