To determine which country became a member of both the Southeast Asia Treaty Organization (SEATO) and the Central Treaty Organization (CENTO) between 1954 and 1955, a brief overview of these pacts is necessary:
Considering these historical developments and alliances, Pakistan was the nation that joined both SEATO and CENTO during the specified timeframe. This dual affiliation was a key component of Pakistan's post-independence foreign policy, seeking alignment with Western nations.
Consequently, the definitive answer is: Pakistan