From the article salmonmac linked to:
"Be sure to treat each of these stitches that are made of two strands of yarn as ONE stitch on the following row or you’ll wind up increasing by accident."
I've learned to use a stitch marker to remind me of the double stranded stitches. I've not folded the ends like that, must remember to try it. So far I've just done a few stitches with the old and new ends together.
I should never overestimate my basic arithmetic skills.