Pattern Confusion

Hi,
I’m a fairly new knitter and have decided to knit a sweater for my dog. I am using Lion Brand’s Patriot Dog Sweater Pattern. I tried to attach the link, but since this is my first post I am unable to. The pattern can be found very easily by searching the name above. I am at the point at which, after knitting in the round to the leg openings, I am now working the underpiece and back separately in rows. I have followed the pattern and knitted the underpiece and slipped it onto the stitch holder. This is where I am stuck. I know I am supposed to now work on the back. However, the working yarn is coming from the stitches on the stitch holder, and I don’t understand how I can work the back separately if the yarn is coming from the underpiece and will therefore connect the two. So I just have no idea what I’m supposed to do? Apologies if this is a stupid question, I am new at this and am in desperate need of help. Answers would be greatly appreciated.
Thank you!

Welcome!
It’s a good question and this is an adventurous pattern to work on.
http://www.acmoore.com/lion-brand-heartland-patriotic-dog-sweater.html
Cut the yarn from the underpiece, leaving about a 4-6 inch tail to weave in later. Work on the back piece with a new strand of yarn (or the end of yarn that you just cut). Later you’ll join the back piece to the underpiece.