Because of differences in fit and finish, depending on the manufacturer of the pipes, we recommend buying pipes in matched pairs.  Most single pipe part numbers have the pair part number indicated in their description.  Commonly, a pair of pipes has both part numbers combined.  Example: 06-2380 and 06-2381 together as a pair would be 06-2380/1.  You can search for the combined part number or the "root" part number, either way should work.