Thanks for the in-depth explanations Nedkel and Phoenicia!

As for the idea of Main Tank / Off Tank, you should forget this concept, it is bad to limit yourself to it. Think Main Tank is simply the tank that is "tanking right now" or "pulling the boss". It is much better to think of the tank pairs as a 1 entity of tanks, so instead of 1 HP bar, you have 2, instead of just having 1 set of defensives, you have 2, and instead of 1 provoke, you have 2, etc.
Even tho i do like this idea and think it's actually the best available route... but is it doable in a duty finder with another random tank?

My problem is (for the time being at last) when i'm with a random tank; I won't know how he plays, what he's about to do, if he's going to shirk on me etc.

Is there any "unwritten rule" which is generally used by good players in a random dungeon?

To be more clear, ler's say I go in a trial (hard). I'll get another random tank. Let's say he's a very good player. How would he behave? What would he expect of me (in terms of role OC).

Should I just beat the boss as hard as i can without caring of how much enmity i generate (and if i get aggroed who cares as long as i'm not standing with/in front of DPSs)?
Should I just focus on generating damage WHILE helping him to keep the aggro? (Like before but i could use shirk in case i get aggro or provoke->shirk before it happens)?
Should I just do something else?

Of course i expect this might change depending on content (in a fight with many adds the OT probably stack em while the MT keep boss at bay, in a super-strong boss the MT could use skills like cover etc. or OC) but as a GENERAL rule how do the two tanks (should) behave when they do not know each other?