Have 2 chars with hunters mark. One casts on target, works fine. The other tries casting on same target, doesn't work. They should work individually for each.
Seconding this.
I will say, it's actually not totally obvious whether or not the same creature by RAW can be affected by two different instances of hunter's mark.
Combining Magical Effects (pg, 205)
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap. For example, if two clerics cast bless on the same target, that character gains the spell's benefit only once; he or she doesn't get to roll two bonus dice.
My ruling is that they can since, even though they are the same spell on the same creature, they are not "combined" in any way, so the rules for Combining Magical Effects do not become relevant. However, I imagine the easiest way to code the rule is to say multiple instances of the same spell with a duration longer than instantaneous cannot affect the same target.