Wow, I think I've got it!
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.
This was the part I missed; I was afraid to cut the yarn because it didn't say specifically "cut the yarn."
Thanks so much, Ingrid, you rock!
BTW, I had been planning to learn how to knit socks on two circulars -- now I guess I'll have a head start on that, eh?
ETA: And thanks for reminding me about Amy's video. I'm a very visual learner and that was a big help too.