If you read the guides you're referencing, specifically the one in this very forum, you would have your answer. But at the risk of this being another long thread on a done to death issue I'll try to keep it short.
Haste portion: Stacks
Increased DOT damage portion: Does not stack
If you have not figured out the Dark Intent priority reasoning from the above facts, then I highly recommend you re read, or reread, Gherkins post entitled "Dark Intent - The Guide" at the link below:
http://www.mmo-champion.com/threads/...82010-12-24%29