This is something I've been wondering for a while but haven't been been able to find the answer to. Does the enhanced dual wield from items such as ninja chainmail and iga zukin +2 count towards the equipment haste cap for delay reduction?
Haste is haste and Dual Wield is dual wield. Neither counts for the other, though both count for the overall delay reduction cap.
If you have 25% haste and 50% DW, your total delay is (1-25%) * (1-50%) = 75%*50% = 37.5% of your starting delay. You cannot go below 20% of your starting delay with any combination of haste and dualwield.
All types of haste are additive, and each type of haste has its own cap (25% for gear, 25% for JA, 43.75% for magic). DW does not directly affect any of those values or caps. However for a given amount of haste or DW, you can determine what the maximum useful value of the other one is based on whether they combine to reach the 80% delay reduction cap.