Dual Mastery was a passive skill exclusive to AP spenders. It could be obtained via the G's Officer Line Capsule, G's Tamer Set Capsule and G's Battle Maid Set Capsule. It came in 1 / 7 / 30 days versions or a permanent version. Some time later Dual Mastery was removed from the loot table of these capsules. You can receive this skill from daily presents.