I've never tried it, but here's a link to directions from UGA. Hope it's what you need.

Here is a link that might be useful: Dividing Ferns

"Often times, ferns such as Boston and sword outgrow their pots; then you have to divide or discard them. When it comes to dividing, ferns can take rather harsh treatment. In some cases, you may have to use considerable force to remove the plant from the pot. Once it's free, use a sharp, long-bladed knife to halve or quarter the root ball. Then, pull each quarter or half apart so you can spread the roots in the new soil. The main requirements after dividing are to water the roots and new soil thoroughly and to provide a humid atmosphere by misting the first few weeks."

That was from the link. I have done it, and am not sure it is worth the trouble. The roots are very dense and you need somthing very sharp and powerful to separate them. Most of the roots and old stalks are dead and are very hard to cut through. You can try it. What happened to me is that I had to abuse the plant so much to get it separated that I only had a few good stalks left. After a few months, it was a full plant again.

If they're big and healthy you can put them on the ground and split them with a shovel. That's all I use and i never lost one yet.