Keyhole scarf

I am a beginning knitter. My project is a simple rib knit with a keyhole. At the keyhole, the instructions are “work in rib pattern over first 15 stitches, join a second ball of yarn,work in rib pattern over last 50 stitches.
Working both sides at once, work even for 13 more rows.
work in rib pattern over first 15 stitches, with same ball of of yarn, work in rib pattern to end. Cut second ball of yarn.”

Is the join just like I have done before, essentially start knitting with a new strand? When I “work the rib pattern over last 50sts” is that with the new ball of yarn only? Does working both sides at once mean alternating balls of yarn each row?

Thanks for any help. By the time I get to the keyhole in this project it will be Dec. 25 and no help from the LYS available.

Welcome to KH!
Work the first 15sts with yarn ball #1 and the remaining 50sts with yarn ball #2. Yes, alternate yarns for the 13 rows of the keyhole so that you have two separate panels.
On the joining row, work across all sts with yarn #1 and continue with that yarn. Yarn #2 can then be cut leaving a 6 inch tail to weave in later.

That makes sense. I feel like I can give it a try!