My friend and I were fooling around with our hunters the other day, and we found out you can get 2 hunters marks on mobs
we were grinding, and happened to hunters mark a mob at the same time, and it worked
its simply done by having both hunters, hunters mark at the EXACT same time
and yes, we have done this several times, so it was not a one time thing.
*this has not been tested for pvp*
but it is a pretty useful thing for farming/grinding cuz it gets the mob down faster
EDIT: This also has not been tested by more than 2 hunters!! not sure if it will work with 3 or even 4, i encourage you to try it though!