The only time things can get tricky when knitting two socks at a time is when you do the heel flap, turn and gusset shaping. That's why I get out my trusty dpn's or skewers and place one cuff on hold while I work the heel flap on the other sock. Now, that doesn't mean you can't do the two heel flaps and heel turns at once, because it is totally do-able. In fact, I'll do them that way depending upon my mood or if I'm a car and I don't want to transfer the stitches, or for whatever reason that pops up. I'll show you how I do it one at a time here. Note the one completed heel on hold in the picture below.
Working a heel flap works the same as if you were using double-points, except one half the sock stitches are placed on one needle for the instep and the other half are placed on a needle for the heel. Once the cuff is the correct length, with the wrong side facing you, begin working your heel flap on one needle only, the one that you designate as the "heel" needle. Continue working your heel flap on one needle only (with the other needle dangling and unused) until the flap is complete. On the next wrong side row, begin to turn your heel the way you normally turn it (using short rows, typically). Once that is complete, you will have one last right side row to finish.
Work to the end of the last right side row to complete the heel turn, then, using the same "heel" needle, pick up and knit stitches up the gusset to the "instep" stitches. Drop the "heel" needle and knit across the instep stitches.
To complete the process, drop your "instep" needle and pick up the working side of the "heel" needle and pick up and knit stitches along the free gusset down to the heel. Then, knit the stitches on the "heel" needle to the last three stitches and make your first gusset decrease, usually a k2tog, then k1. Drop your "heel" needle and then knit the instep stitches on your "instep" needle to the end. Drop it, then, using the working end of the "heel' needle, k1, then ssk to complete your first decrease "round" of gusset shaping. Continue working this way and work your gusset decreases on every other round until the number of heel stitches match the number of stitches on the "instep" needle.
Work one round even and then place these stitches on "hold" and work the other sock on the two circulars as before. Once both heels are complete, carefully place the on-hold stitches back onto the two circulars in tandem as you did with the cuffs and work the body of the sock until you are ready to make the toe decreases, which are done at the same time and are super easy and fast to do when the two socks are worked at the same time.
There is no hard and fast rule about how to separate your stitches. Some guides will tell you to place one half of the instep stitches onto one needle, and to place one-half of the heel stitches on that same needle with a stitch marker to separate them. For me, it makes more sense to devote one needle for the insteps and one needle for the heels and to remember to make decreases for the gusset one stitch before the beginning and end of the "heel" needle, but if it makes more sense for you to use markers and separate the two sides, go ahead and do that, instead.
Stay Tuned for "Knitting Two Socks at a Time: Aren't You Glad You're Finished?"