You can use a dorn-dergar with only one hand.
Benefit: You can use a dorn-dergar as a one-handed weapon. When using it one-handed, changing whether it’s a normal or reach weapon is a full-round action. If you have the Darting Viper feat, changing its reach is a move action.
Normal: A dorn-dergar requires two hands to wield.
Pathfinder Player Companion: Adventurer’s Armory 2 © 2017, Paizo Inc.; Authors: Jenny Jarzabski, Mikko Kallio, Isabelle Lee, Luis Loza, Joe Pasini, David N. Ross, and Linda Zayas-Palmer.