There are no countries with official short names that are only two letters long.
When referring to countries in a shorthand or abbreviated manner (for instance, in domain names or international vehicle registrations), two-letter codes are often used, but these are abbreviations and not the official short names of the countries. The International Organization for Standardization (ISO) provides a list of two-letter country codes (ISO 3166-1 alpha-2) for this purpose. However, these are not the official names of the countries; they are just codes.