How exactly does it work?

I'm not getting how does it work... does it work like a talent? So if I lose the Druid talent I get to have the same bonuses by wearing it?

Based on the description, I'm assuming you just get 50% more training exp for druid-related skills, even when you aren't druid talent.

It works like a reforge, not a talent.

i want to know if you can wear two of them at once

I've seen people wear two different ones, never 2 of the same though.

Define different?

Sage of the Night Badge and druid's mark for example.

I'm pretty sure wearing two druid's marks will result in the same bonus as wearing one. The item's description itself already says "If multiple badges are equipped only the effect with the highest bonus value will be applied."