If there is nothing wrong with the ballast and or wiring it could be a bad socket? I believe HD or Lowes sells these if not than electrical supply houses sell them for around $5 if they are not WP. Sounds like you have a rapid start ballast which helps lamps last longer compared to the instant start ballast, but if it runs two lamps and one goes out they both go out or one looks dim. Like aggiereefer said try changing ballast first and go from there.