Hi and welcome to knitting and to Knitting Help!
Yes, you should cut the yarn after you cast off for the back. Leave about a 6 inch tail so that you can weave it in later when the pieces are put together. It's very common for patterns to be written in several pieces. But because we like to knit and not so much to sew things together, there are also patterns that are knit in one piece or just a few pieces. Good luck with the sweater and come back if you have more questions.