With some interest in knitting with two circular needles. I wanted to give some tips and tricks on how I handle it. These are my Thermal sleeves knitting on 2 circ's and I'm already finished the increase rows and just moments away from the armhole bind-offs.
Cast on your desired amount of stitches. For this example we’ll use 20. Divide said number of stitches in half by slipping 10 stitches on to the second circular needle.
Knitting two pieces at the same time? Slid the the first cast-on to the end of the needles, so the cast-on tail and active yarn are facing to your left. Grab the circular needle that also has the previous cast-on tail. Using this will ensure that the pieces are both mirroring each other. With second ball of yarn cast-on 20 stitches for second piece.
With second needle slip 10 stitches dividing the cast-on in half.
Turn work and making sure not to twist stitches. The Cast-on bumps should all be facing downward. Start knitting your pieces as instructed by your pattern. For example: Knit2, Purl2 rib.
Taking careful actions to knit with one needle for one side only. If you happen to knit with both needles, the project will become tangled. Also, be careful to switch balls of yarn between different pieces.
Once one side row is complete, turn work and using second needle work second side row. Below are some graphics used to explain the needle path with for two pieces.