Okay, so here's how I split my STR yarn so I could have 2 balls of yarn to do Magic Loop:
First, I weighed the yarn. Although it said 5.5 oz on the label, it's weighing in at 6 oz, so I want to split this into two 3 oz balls of yarn.
Here is the STR yarn happily coiled around the swift. That's felt I'm using with the clamp so it doesn't damage the table.
I wind my yarn using a Nostepinne or Nostepinde. (I've seen it spelled both ways). I bought mine here, but many people have had success using medicine bottles, toilet paper rolls, broken off legs of chairs, etc. You start by winding the yarn in a horizontal spiral, and then gradually tilt the Noste so the yarn is wrapping diagonally. You then start turning your Noste away from you; counter clock wise. With practice, you can shape the ball however you like; I like mine to have a flat base so I don't have to contend with balls rolling under the couch to the nether regions of Dust Bunny Land from which few things return unscathed.
In this case, I kept winding the ball until it reached 3 oz (once in a while, I would take the ball off the Noste and weigh it to see where I was.) Once it got to 3 oz, I looked for the same area of color where the ball began and cut the yarn there. This way, both balls of yarn start with the same color pattern repeats, which is good if you're using self striping yarn and want your socks to match. If you're nervous about cutting the yarn, leave a long tail off the end of the first ball and then start winding the second. When you're done, weigh both balls to make sure they're the same, then snip!