If you are going to knit two-at-a-time socks, you'll need 2 center-pull balls, of course.
The good news is, in order to create a center-pull ball, you don't need any equipment. Other than your hands! But, that means you have to finish the job all at once, and you can't stop halfway to answer the phone or cook dinner. So be warned.
Here's how to do it, equipment free. This video doesn't show what to do with the hank/skein, though. My advice: hang it around the back of a chair so it doesn't turn into a tangled mess as you're winding it off. Or, sit on a chair and lay it across your lap, with your knees poking through the center of the hank. Or better yet, recruit someone to stretch the skein over their two hands to help you out.
If you have a 100gm hank or skein that you are trying to divide into 2 balls, invest in a cheap kitchen scale ($25 or less). Weigh the wool before starting, and then periodically reweigh to see if you've got halfway yet. Beats counting windings.
If money is no object, you can spend big bucks and get yourself a ball-winder.
[ball winder from http://www.woolydesigns.com/]