Flap heels for 2-at-a-time, top-down socks

OK, you've cast on two socks, and you have knit tubes that are as long as you want your socks to be (up your leg). Now you are going to knit the heel flaps and turn the heel.

To knit the flaps, you will be going back and forth on one side of the two tubes. You should use a chain-selvedge along both sides of the flap. Knit across one sock, then across the other, then turn and go back across the two. In this way, they will both be exactly the same length.

[half-way done the heel flaps. Note chain selvedges on both sides of flap, and slipped stitches for reinforcement. Click picture to enlarge.]

This is a good time to think about reinforcing the heel, because it sees a lot of abrasion. You can:
1. use some kind of reinforcement thread (wooly nylon serger thread or official reinforcement thread - but NOT polyester sewing thread which will actually cut into your yarn over time) - just hold it together with your yarn and knit as one,
2. use slipped stitches to create more thickness in the knit fabric ("eye of partridge", slipped stitch ridges, or linen stitch are all examples)
3. knit the flap with a different type of wool with tighter ply and/or higher nylon content (often done commercially - think of those grey work socks with red heels).

Once you have knit the flap to the desired length (I do a minimum of 2.25" for ladies' socks, 2.5" for men), you are ready to turn the heel (or use your favourite from Knitting Vintage Socks). You will turn the first sock's heel first, ending with a RS row. Then pick up and knit stitches along the side of that sock's flap that is closest to your needle, and then move on to the second sock to turn its heel, as shown in the graphics below (note: the orientation of the diagrams is as if you put the work down on the table in front of you, with the flaps at the back. Just like in the photo above. It's not the view you have while knitting,it's the view of your onlooker.) :

[1st sock, leftmost, with turned heel. Pick up & knit sts along flap - red arrow]

[Now you turn the heel on the second sock, as per the arrow.]

To avoid the common hole at the corner of the heel flap and instep, you can knit a stitch out of the cross-ways yarn right after you've picked up all the side sts, before the first instep stitch. Working it TBL, as shown in the link, closes the hole.

When you're finished turning that heel - ending again with a RS row - you are ready to pick up sts along the same side of the 2nd sock's flap:

[Done turning both heels. Pick up flap sts on 2nd sock, as per arrow.]

Again you can pick up another twisted stitch to prevent a hole if desired. When that's done, knit across the insteps of both socks.

Now you pick up the sts on the other side of the 1st sock's flap.The hole-preventing extra stitch is now the first thing to do, if you think you need it!

[ready to pick up remaining flap sts on leftmost sock...]

Now knit across the heel stitches and down the other side of the flap, and you are ready for the second sock. (oops, I notice a mistake in the picture below. The working yarn for the second sock is pictured in the wrong location! It should be at the bottom of the flap, having just finished the instep sts. My bad.)

[Almost done! Now pick up the remaining flap sts on the 2nd sock.]

Once you have picked up the last flap's stitches, you can start knitting in the round again. I usually knit the first row of flap-side stitches TBL, twisting them to tighten them up.

There are many ways of picking up the flap stitches that will work, but this particular sequence ensures that all the instep stitches end up on one "side" of the circular needle, and all the gusset+heel stitches are on the facing "side". This configuration is not a requirement for knitting the rest of the sock - I know, I've done it differently a lot of times! - but this gives the most logical and straightforward setup and leads painlessly into the toe construction.

That was the trickiest part of knitting two-at-a-time top-down socks! Now you just continue to knit in the round, completing the gussets, the rest of the foot, and the toe. You can fit as you go.

I start the toe decreases when the sock hits the bottom of the nail of my little toe (try it on!).