I find that when embroidering mirror image designs, I always get a better result when I stitch both sides roughly at the same time... if I (as I would like) move on to something else and then come back to it days or weeks later - I am less likely to get the pair looking the same. 

I always prefer to work on colour selections during the day and with this grey scale it is even more important as the tones of the grey yarns change completely at night.