You certainly can do that. You just need a different ball for each side. When I do two sleeves at the same time (if I do one at a time, I resent the second one
), I always put a marker on the needle between the two. This way I'm sure to do both on each row, rather than going into autodrive and working on one and forgetting whether I did the second.
At the point when you finish one and are about to start the other, you'll have one piece on each side of the points, and if you put it down, it's easy to forget where you were.