Open one door of a double door and the other one opens with it. Close it and they both close. The same goes for a hatch made of trapdoors and for fence gates stacked on or beside each other.

assorteddoubledoors:ignored_by_double_doors tag always open on their own, for modded doors with rules of their own.These are in the assorteddoubledoors-common config file.
doubleDoors.doors opens both doors of a double door together. On by default.doubleDoors.trapdoors opens every trapdoor of a hatch together. On by default.doubleDoors.fenceGates opens fence gates next to or on top of each other together. On by default.doubleDoors.maxConnected is the most trapdoors or fence gates one click opens. 16 by default.Double Doors also comes in Assorted Util with the other five Util mods, if you'd rather have them all in one download.
The full guide is at assortedmods.com. It's also in the game, in the Instruction Manual from Assorted Lib.
Found a bug? Report it on GitHub.