Knitting front side of shirt adding sleeves

by Jennifer
(New Mexico)

Pattern is knitted in two pieces, front side and back side exactly alike and stitched together. I am at the point where I start adding sleeve and am confused by the instructions. At this point I have been knitting the front side on size 8 needles. For the sleeves they want us to use size 10 needles. I am confused as to the “desperate into two equal parts“ are the sleeve parts only knitted on size 10 and the body is kept on the size 8 or what? Please help! See instructions below:

Row 19 - all knit, separate into 2 equal parts which will be worked on long straight needles- one side at a time.
Row 20 - cast on 20 st. at the beginning of the row ( one side of the sleeve ) , all purl
Row 21 - cast 20 st. at the beginning of the row ( second part of the sleeve ), all knit