The pattern says: "[SIZE=-1][FONT=helvetica,arial,sans-serif][FONT=helvetica,arial,sans-serif]You can now knit both sleeves at the same time..."
Can means you're able to, you don't have to. To knit both at once, turn and begin working across the sleeve stitches; at the end, drop that yarn and pick up a new strand and knit across the other sleeve. That's it - you just have to remember to use the separate strands for each sleeve. You can also just do one and finish it, then work on the other. They can be knit flat and seamed up, or you can put one on a stitch holder too and work them around. Or leave one of them on the needles and use dpns to work the other, then come back to work on the other. It's not specific because it's up to you how you want to do it, which ever way you find easiest.