I’ve done 2 at once both with 2 circs and Magic Loop and I’ve always used 2 balls. I found that when I use Magic Loop, the tangling is much less than 2 circs, because the yarn gets tangled in the extra circ also.
I used this pattern for the socks, Two At Once, Toe-Up Sock Pattern and I used the Turkish Cast-on, here. The sock pattern is for 2 circs, but I adapted it to Magic Loop. I had to experiment with the Turkish cast-on to get it to work, but if you do the wraps for both socks first, and then start the knitting part it works out. It helps to practice first.
There are pictures on my Flickr that show how the socks were on the needles, but they are not all together. I guess I should do some tags or something.