I have been away from documentation for a long time so I am not confident in my writing abilities, but let me give this a whirl.
1) Get your Yarn and Needles Ready! I like to use a 40 inch circular needle, although a slightly shorter needle (such as 32") works as well.
2) Cast on all your stitches for the first sock (mitt, sleeve etc) on one needle. Let's call this Needle A.
3) Pushing those stitches back off the needle you will count exactly half of those stitches. Then pull the loop through the halfway point. Now you have half of the stitches on each needle. This is where you would join in the round if you were only knitting one sock. Let's now cast on the second set of stitches.
6) Now your leading strand is at the back of Needle B (rather than at the tip). Continue to cast on the second half of your stitches on Needle A.
Here is One stitch cast on Needle A.
Here are 5 stitches cast on.
Here are all stitches cast on, half on each needle.
7) Join in the round, remembering not to twist your stitches. At this point, you can refer to more information on Magic Loop knitting. There is a You Tube by Knit Witch you can refer to as well.
8) Remember to swap leading yarn when you reach the second sock. Otherwise you will attach the 2 socks together, or learn how to tink. I've done it, I know.
I hope this is a useful tool. All comments are welcome.