P and slip to holder means for you to purl the stitches and slip them to a holder to be worked later. You can use a regular stitch holder or a piece of yarn pulled through the stitches with a large-eyed needle.
Trust the pattern on this one.
You'll work each side with it's own ball of yarn, but they're worked one at a time so you won't have to use multiple strands at once.
What you're doing is making each side of the center collar longer to reach up to the shoulders. You won't work the center of the neck until the very end. You'll pick up stitches down the edge of one side of the neck, work the stitches of the center and the pick up stitches up the other side and work them in the ribbing.
There won't be any seams, and it will all look like a continuous knit.
As I said, just follow the pattern step by step and it will work out fine.