Here is how I split a 100 gram skein of Sock yarn into two balls in order to knit a pair of socks at the same time and have the striping and/or patterning match up.
Weigh the skein without the ballband. Divide the skein weight in half (48 grams).
Find both yarn ends. Pull out the inside yarn end and create "yarn barf" by pulling out half of the skein ~ 48 grams. Try not to handle this mess too much so it doesn't tangle around itself.
Take the outside tail from the original skein and the middle-ish yarn connecting both "balls" and find a section where colors and patterning match up. Take your time it's a bit confusing. Cut and separate the skein.
Since I knit from the outside of the skein I keep the matched up colors on the outside of the ball.
Create two center-pull balls and remember to wrap the yarn very loosely so the yarn isn't being stretched.