It’s a bit tricky to grab evidence for every single part right now, so I've written out the explanation based on my experience to get the ball rolling. I’ll make sure to update this with the actual documentation as soon as I get it.
The Druid itself count as secondary target on a duo party?
Yes. In a party of 2, 3, 4, 5... to infinity, as long as the Druid doing the healing has the lowest health, they will be targeted as the secondary target.
The secondary lowest health target its based on the actual hitpoints left or based on a % based on the full life?
It's based on the actual number, not on the percentage of missing health. For example, if a Knight is missing 80% of their life but still has 5k hitpoints, and a Sorcerer has 3.6k hitpoints (they do need to be missing some health), if you heal a third person—let's say the Paladin—the secondary target will heal the Sorcerer instead of the Knight.
What happened when two secondary targets have the same hitpoints?
I haven't been able to test this yet. I don't want to guess, so I will provide an answer in the future.
The secondary target can be affected for critical sio too?
Yes, both heals can critically heal, even at the same time.
