if you have two circs the same size you won't have to use DPNs, you can do the decreased portion on two circs. the video is on the same page that Jane linked.
As far as the join goes, what i do is bring the two points together after i have cast on, and swap the first stitch and the last stitch and then start knitting in the round like Amy shows in her video. helps to tighten it.
i also used to cast on one extra stitch and knit the first and last stitch together... I found that swapping is easier, though a little precarious when you pop those stitches off the ends of the needles!