Would the benefits from the feats: Two-weapon Fighting, Two-weapon Opening, Two-weapon Defense, etc... apply to a character that uses a Double Axe, Double Sword or Double Scimitar?
You get exactly the same benefits as if you were wielding two one-handed weapons.
Wielding a double weapon is like wielding a weapon in each hand. [Adventurer's Vault 10, identical in online Compendium entry]
There are a couple of weirdnesses with regard to Small characters, the
stout keyword, and so forth, but as D&D is an exception-based system and none of those exceptions seem to be commonly prone to confusion, it would be tedious and unnecessarily listy to go into them here.