Even if they worked together, you can't gain double combat advantage. The bouses don't stack. It does, however, give you two separate ways to generate combat advantage, with Bluff, in the same encounter.
Bluff said:Gain Combat Advantage: Once per combat encounter, you can try to gain combat advantage against an adjacent enemy by feinting. As a standard action, make a Bluff check opposed by the enemy’s passive Insight check. If you succeed, you gain combat advantage against the enemy until the end of your next turn.
Changeling Trick said:Effect: You make a Bluff check opposed by the target’s passive Insight. If your check succeeds, you gain combat advantage against the target until the end of your next turn.
Changeling Trick grants you an ability. The Bluff skill grants you an ability. To say that use of one ability also uses up the other ability would be a DM levied nerf. You clearly have two ways per encounter to be granted combat advantage.Skilled Feint said:Benefit: You gain a +2 bonus to Bluff checks made to feint. In addition, when you make a Bluff check to cause an enemy to grant combat advantage to you and succeed, your Sneak Attack deals an extra die of damage to that enemy until the end of your next turn.
Well, that's part of the problem. Changeling Trick is a minor action, whereas a Bluff in combat is a standard action. So I can see people arguing that Changeling Trick is just giving you the same Bluff action as a minor instead of a standard.