First, since they say to 'join double strand of yarn' you'd cut the yarn you've been using and restart at the end of one of the circs.
You're working the body in the round on two circs--basically you slide the needle so one point is ready to knit, and you knit the stitches on that needle with the other end of that needle. Then you pull it so the stitches are on the needle cable and turn your work and do the same thing on the other side.
That will join them in the round.
Amy has video on knitting with two circs in the Advanced section under circular knitting.
"Trust the pattern!"
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IngridKH on Ravelry