Snowbird - Getting its Wings
Since it's been a while since I've knit anything flat, I'll explain why I like to knit flat sleeves at the same time. Most of the time, sleeves are identical in their shaping, so by knitting them at the same time I not only have an identical gauge from one sleeve to the other, but all shaping (and shaping mistakes) should be identical. The end result is two identical sleeves. You can also use this with fronts on a cardigan although cardigan fronts aren't identical, they're mirror images of each other. The only thing you'll need to work two pieces of a sweater at the same time is a second ball of yarn and a bit of patience because sometimes the yarns get wrapped around each other a bit.