I'm making my way down the foot of Nancy Bush's Knot Socks from Sock Knitting Master Class. Recently, I was asked how to make two socks the same length so I thought I'd show you what I do.
As I work my way along the foot, I place a safety-pin type marker in every 20th round of knitting. This way, I only have to count 20 rounds at at time, then I place a final marker on the round before the toe (or heel) begins. Then I know exactly how many rounds to work for the foot of the second sock. I've found that I try to knit at least to the next marker at each sitting, which makes me finish quicker. I do the same thing when knitting the legs.