I'm knitting a pair of Kilt Hose for my son. I've got 2 socks on 2 circulars, the ball of wool is a natural color, and I'm working it from both ends of the ball. The ball is parked on the Nostie the center pull yarn is attached to one sock, the other yarn that unwinds from the exterior of the ball, is attached to the other sock. When it is time to 'flip' the needles, I simply 'flip' the Nostie...the yarn never tangles.
Here, I have set up 2 balls of yarn, each on its own Nostie. I am currently working a swatch on the one shown here. To work 2 socks from 2 different balls of yarn, park each ball on their own Nostie. The center pull is attached to the knitting work.
Here, I have a commercial yarn 'parked' on a Nostie. (Yes, I sometimes have commercial yarn to work with)
I was swatching with the needles, so I had them detached already from the work, but you can see that the pull center is coming off the working end of the Nostie, going to the needles and the project.