In most cases, I cast on and start the second sock as soon as the first is completed. In the case of the socks I’m knitting from Wacky Windmill yarn, I got distracted with other projects. And that’s a shame because I really love the yarn (my photo does not do justice to the rich coppery colorway) as well as the pattern (I’m following the stitch pattern for Seeded Rib Socks on page 88 of Getting Started Knitting Socks — but at 8 stitches/inch instead of 6 stitches/inch). But I’m back on track now and have knitted about half of the leg.
The first sock is for reference — I always count rows to make sure the second sock matches the first. The markers are placed every 20 rounds along the foot, with the last one placed on the row before I begin the toe shaping. I find psychological comfort in placing markers as I go so I can see my progress.