The main problem with doing a left and right pair has nothing to do with making them look alike. The problem is that on one, for me the left one, everything is done backwards. For example I like to draw the carving to the rear of the cheek piece from left to right but on a left hand rifle, the flow of the rifle would be more natural if you drew it from right to left. Likewise with the stock shaping, etc. It's really difficult if your shop isn't set up so that you can work on either side of your vise with equal ease.
Tom