To be honest, it's probably NaB, but for a modern game, it's definitely bad behavior. Using this two ability breaks logs allowing either suffering or abuse of a wrong calculation. Also, this behavior accelerates the approach to the bound (3 stacks).
So the expected behavior is that each druid will apply its own HoTs effect!
1. Current Behavior
1.1. Description
At the moment, if two druids give Tranquility, you will only see one HoTs effect. They will update the first effect!
1.2. How to Reproduce
1.3 Source Material
https://classic.warcraftlogs.com/reports/Xr3tVgRfmGjJ9q7b#fight=9&type=auras&start=2576322&end=2613046&pins=2%24Off%24%23909049%24expression%24isTick%20%3D%20false%5E0%24Separate%24%23909049%24auras-gained%240%240.0.0.Any%240.0.0.Any%24true%240.0.0.Any%24false%24740&ability=44203&view=events&source=5&translate=true
The last tick of the second druid updated the HoT effect of the first druid. All healing will be credited to the first druid.
2. Expected Behavior
2.1. Description
To be honest, it's probably NaB, but for a modern game, it's definitely bad behavior. Using this two ability breaks logs allowing either suffering or abuse of a wrong calculation. Also, this behavior accelerates the approach to the bound (3 stacks).
So the expected behavior is that each druid will apply its own HoTs effect!
2.2 Source Material
https://wowwiki-archive.fandom.com/wiki/Patch_2.0.1 HoTs from different sources will now stack (i.e. multiple druids can cast the Rejuvenation spell on 1 target).