If the serial # are the same except they end with A and B, this tells you they were originally sold as a pair. Some manufacturers "match" drivers and even tweak crossovers, or it could be the finish is matched. It at least designates that the speakers were manufactured at the same time which is good as this avoids fluctuation in component quality or minor changes to design.
If the drivers are offset, usually the tweeters, there may be a recommendation concerning L+R but usually the user is advised to experiment to see which way they work best in their room. It's really up to you.
