I've never divided BOP, but in general, a rooted plant needs the leaves on so it can photosynthesize. If the plants are out of proportion, way more leaves than roots, you can cut some back, but not all of them.

I have split up giant white BOP many times, they seem to take the abuse rather well as long as they have a few weeks out of the sun and wind to recover from transplant shock. I grow them in pots so I can easily situate them in a protected area.

If the plant cannot support the leaves, the older leaves will wither and dry up on their own, so don't hack too much because you may end up robbing the plant of metabolic potential.