Double knitting
Paired front and back stitches make two fabric layers at the same time.
Counted practice swatch
Cast on 20 pairs: 40 stitches, alternating colour A and colour B. Keep the yarns together when moving them front or back.
Work the repeat
Each pair: move both yarns back and knit the front stitch with A; move both yarns forward and purl the back stitch with B. At the edge cross the yarns. On turning, use B to knit the new front layer and A to purl the back layer.
Read the fabric
A two-colour chart marks pairs rather than individual needles loops. Reverse the colours within selected pairs to make a mirrored motif on the second face.