Many of my students claim that ribbing and double knitting Portuguese Style has a rhythm that makes the work easier and more pleasant. Again, it requires less maneuvering of your hands when you work with the yarn available in front of the work at all times.
Double knitting implies that you are producing two knitted fabrics parallel to each other at the same time. For instance, you might choose double knitting when you knit a pocket on a sweater or when you want to line a mitten for extra warmth. Producing the garment and lining at the same time avoids sewing and finishing it later.
When working two-color double knitting patterns, each square of the chart represents two stitches (pair) and the pattern is reversible. If you are knitting a motif from a chart, the same motif is worked on both sides at the same time but the colors are reversed. For each pair of stitches you knit the first stitch with color A and purl the second one with color B (if you do not have a motif you only have one color for both stitches).
For detailed information about the technique I recommend the book Double Knitting Reversible Two-Color Design by M’Lou Baber.
How to work a pair of Stitches
NOTE: In the pictures below I am working circularly and I cast on using two strands of yarn. After that I did ribbing 1X1 for a couple of rounds. When you start double knitting you have a pair of sts to work with.