I believe the double light sabers showed up in the books long before the Phantom Menace...
I seem to remember someone telling me about one of the characters having a saber with multiple settings... one of them being double sided.
Exar Kun apparently used a double sided lightsaber. But I can't remember if he was introduced prior to Phantom Menace.
Exar Kun was introduced in the old Tales of the Jedi: Knights of the Old Republic comics.Exar Kun apparently used a double sided lightsaber. But I can't remember if he was introduced prior to Phantom Menace.