As stereo recording is among the main applications of small diaphragm microphones, they are often offered as matched stereo pairs. "Matched" meaning that the manufacturer selected two microphones that are very close in sensitivity and frequency response. Close matching is important for achieving a symmetrical stereo image. Some manufacturers will give you a matched stereo pair at the price of two individual microphones (sometimes even a little cheaper), others charge you for their matching service, so the pair is more expensive than two individual microphones. However, the latter manufacturers - often the more exclusive brands - take greater care and use better measuring equipment for their matching service. So the extra expense is not unjustified.
Keep in mind that it is very difficult, if not impossible, to obtain a second microphone that exactly matches a mic you already own. So, if you plan on making stereo recordings, get a matched pair right away.