General Flag Rules do not specifically address the positioning of a flag patch; a decision is left to the discretion of the organization prescribing the wear. Some elect to use the "left" flag on both sleeves. If you are planning to wear only one patch, it is recommended that you wear a "left" flag on your left sleeve.
Military guidelines: To wear our country's flag properly, the field of stars is worn closest to your heart. Further, when worn on the sleeve, the flag should appear to be advancing and not retreating. Thus, if your flag is to be worn on your LEFT sleeve, use a left flag. For flags worn on your RIGHT sleeve, use a "right" or "reversed field" flag. Source: US Army web site FAQs.