Trendy sleeves and a matching cowl with an intarsia detail:)

Intarsia is generally worked flat and then the pieces are sewn together: depending on the sewing method you can decide to add or not a selvedge stitch.
There are some techniques that allow you to knit in the round (where you are still actually knitting with the flat technique and using joining methods). In this case you can use circular needles.
If you are knitting flat you might think about knitting the two sleeves together or one at the time.