I ALWAYS do my sleeves together on a circ!! Life is so much easier that way. :D This way you know they are the same length and that your increases are on the same row. The only thing I would recommend is to put a stitch marker between the two sleeves to prevent you from 'forgetting' to do the second sleeve and automatically turning and working the same sleeve an extra few rows. Don't ask me why I found it necessary to come up with this idea.