Imagine a druid with Subtlety 5/5 hotting a pally with Stoicism 2/2.

Subtlety: Reduces the threat generated by your healing spells by 20% and reduces the chance your healing spells will be dispelled by 30%.

Stoicism: Increases your resistance to Stun effects by an additional 10% and reduces the chance your spells will be dispelled by an additional 30%.

Does anyone knows if the hots would have a :
- 60% reduced chance to be dispelled (the talents stacks additionally)
- 39% reduced chance to be dispelled (the talents stacks multiplically)
- 30% reduced chance to be dispelled (the talents doesn´t stacks)

Agreed. It says "Your spells" not "Beneficial spell effects on you". So the talent would only count for what the paladin himself has cast, as well as only on what the druid has cast. Better to askthis inside one of the pally/druid threads in my opinion.

That's my understanding as well, but it's possible that Lifebloom's a special case. Since Lifebloom benefits from the druid's +heal but is credited to the recipient, it might end up getting some odd mix of other talent effects. I don't think it's too likely, though--Lifebloom doesn't benefit from both the druid's and target's +heal, or at least I haven't noticed it doing so.

I'm not sure how you'd go about testing it ("A druid, a paladin, and a shaman walk into the Dire Maul arena...").

I believe you're thinking of Prayer of Mending. I distinctly recall seeing a healer's name attached to lifeblooms on me in SCT.