You will put the 16 st. on the two needles together onto one needle. So it goes like this,
Pick up your 16 sts then with a new needle knit the two 16 stitches that you put onto one needle. With a new needle pick up 16sts on the other side of the heel flap.
you are now up to the top on the opposite side of heel flap. Knit half of the stitches together with the 16 side stitches you just picked up. I put a marker between these to show the new start. I know this funny but just trust the pattern and you will get it right. The first time I got to this point I was just like you didn't know what to do.
I just kept saying trust the pattern from Ingrid and I haven't stopped making socks since. Love making them for my husband and he loves getting them. They are the only ones that fit his high instep. Custom made is the best.
I was a little wordy I hope this has help you.