When I was looking for sock patterns, I ran across a video of Cat Bordhi demonstrating the "Magic Cast on". It was amazing! I used it for my socks and it worked great, so I decided that I would put pictures on my blog to show how it is done. These show using 2 circulars since that is what I did with my socks.
Hold the 2 needles together (for socks, put the longer one on the top as it will be used for the heels later).
Put the yarn between the needles and up over the top.
Cross the short end over the working yarn in back.
Wrap yarn around thumb and pointer as if you were doing a long tail cast on.
Now you are going to add the stitches. Each wrap is a cast on stitch.
Bring the yarn coming off the pointer around the bottom of the lower needle and around the front and through the middle between the needles.
Now bring the yarn from the thumb between the needles and around the front and behind the needles. (It is the same motion as you did with the other needle)
Repeat the two wraps until you have enough stitches.
Then wrap the short end over the working yarn in back.
For two at once socks, slide the stitches over and do another set of cast on stitches for the 2nd sock. Make sure that the fronts are on the same side!
Turn the needles clockwise, so that the bottom needle is now on the top. Pull the new bottom needle through to the center of the cable. Start knitting with the other end of the circular needle on top.
Here it shows the front after you've knit across both sides (with both needles)
And here is the back. See the Purl bumps? You can't tell where the cast on is! Cool, huh??