Double knitting is a technique of knitting two pieces at the same time, wrong sides together, by alternating stitches between the two pieces. If you switch the working yarns back and forth between the two pieces, it will join them together into one reversible piece, like this reversible blanket piece. If you carefully keep the two pieces separate, it gives you a pair of separate, mirror image pieces like this pair of socks.
In either case, the stitches alternate between the front and back pieces as shown in the picture below. The red (front) piece is worked on the right side, and the brown (back) piece is worked on the wrong side.
To work a decrease, you need to have two stitches from the same side next to each other. First slip one red (front) stitch to the right needle.
Then remove the next brown (back) stitch from the left needle, and place it on a cable needle, locking stitch marker, or some other holder.
Now place the previously slipped red stitch back onto the left needle and work the front decrease as normal. In this case, I am working a k2tog. Note that I’m only holding the cable needle up so you can see it in the picture. normally I would just leave it hanging in the back while I work the decrease.
Next, transfer the dropped brown stitch back onto the left needle, and work the back decrease – remember with double knitting you are working on the wrong side of the back fabric, so in this case, I would work a p2tog.