[use a helper needle and a long, flexible circular needle]
Step 1: Hold one end of the circular needle and the helper needle together, points in the same direction (you will be totally ignoring the other end of the circular throughout this whole procedure.) Cast on over both the helper needle and the circular - this will keep the stitches a bit looser and make it easier to knit the first round. Cast on half of the stitches for the first sock (shown in green below), and then all of the stitches for the second (pink) sock. Place a marker exactly halfway in the second sock.
[half of the first sock, all of the second cast on, with marker halfway.]
Step 5: Now you can remove the helper needle. It has done its helpful job! Then, fold the circular needle over to meet its other end...in the picture below, that means I'm moving the point clockwise (to the right).
[helper needle removed, folding stitches over]
This is what you should have:
Notice that the wool ends are in the same position on both socks. The ends of the circ are off to the right, and the marker to the left.
You start knitting with the wool ends on the right and on the needle farthest from you. Yank the far side of the circ out to free it totally and move the stitches so that the first sock (green) is actually on the needle closest to you so you can knit it. Be careful when pulling on the circ to keep the marker end of the loop from sucking into the stitches - you want the "magic" loop to be maintained. Join, being careful to avoid twists in the stitches. I usually grab both ends of the wool (the remaining tail as well as the end heading to the ball) and knit a couple of stitches with both, this anchors everything.