First of all you can loot more that 1 rares in cases when avaliable items in loot from the boss contains more than 1 rare item. You can not loot 2x the same item. So we can easily say that it is imposiible to loot 2x Eye Of The Storm, but in Case of The Welter when you can loot 2 different items like Triple Bolt Crossbow and Shrunken Head Necklace it is possible.
It works for random spawn bosses with reward chest system.
Raid bosses like Ferumbras or archdemons (or even Pale Count) have a bit different system - here we've got quite a lot of rare items in the loot, but usually get it only one person with highest participation. There is one exception - because there is 100% chance to drop Ferumbras' Hat for the person with the highest participation and rest of rares have slightly different chance and can be dropped by other person with really high participation - we can meet the situation when 1 person is looting Ferumbras' Hat and another one - for example Skullcracker.
I think that this situation is more common in case of bosses like Fermubras Mortal Shell or King Zelos, where chance to loot Ferumbras' Hat or Shadow Cowl is more random than in case of other items like Great Shield etc.
In case of Jaul (because it might be interesting and helpful) - we can loot ALWAYS 2 items from the boss, and it is possible that both of them have one person.
Grand Master Oberon - I think we can not compare it to any other boss. Yes, it is possible to loot, some of players were already lucky enough to proove it:
Old bosses withour reward system like Man In The Cave, Fernfang or Dharalion have only 1 rare item amond the loot list so it is impossible to loot more than 1.