Star Trek: Gemini

Captain James T. Kirk and the Starship Enterprise™ have been sent to the planet Nador to participate in a watershed event: the Nadorians' first true election, to vote on whether or not to join the United Federation of Planets.
Supporting the Federation are the planet's joint rulers: Their Serene Highnesses Abon and Delor, Siamese twins joined at the spinal cord to represent the unity of the different tribes of Nador. But a shadowy group of fanatics wants nothing to do with the Federation, and will stop at nothing -- whether it be assassinating the princes or kidnapping Captain Kirk's nephew Peter -- to achieve their goals!

Kirk must work to stop the fanatics from wreaking havoc on Nador, and from harming his beloved nephew -- but even he may be hard-pressed to stem the tide of chaos when the princes' horrible secret is revealed!