Using Dark Intent on a Lock as a Lock means that you are a moron.
The 3% Haste buff stacks.
The 9% Damage/Healing buff does not.
Therefore, if you cast Dark Intent on a Warlock who cast Dark Intent on you, you each get 6% Haste (12% total) and 9% Damage/Healing (18% total). However, if you each cast Dark Intent on a non Warlock, The Raid gets 12% Haste and 36% Damage/Healing.
I have this data readily available and did not post it because it would be used to compare against non-warlocks, and you can't do this. You would have to compare it to TWO non-warlocks and sadly not everyone who reads this post will understand that.
For those of you curious, for Warlocks swapping Dark Intent to be viable your group must have all Holy Paladin Healers, a Warrior tank who does not use Deep Wounds or Rend, and not have any Priests, Mages, Druids, Death Knights, Rogues, Elemental Shamans, and non-BM Hunters. Basically a group with zero chance of success against anything of moderate difficulty.
So I hope you understand why I'm reluctant to post that information.