The applying multiple hunter's marks refers to having 2 or more rangers in the party with the spell allowing them to both mark the same target. Before only one ranger could mark one target at a time if a different ranger attempted to mark it would remove the first ranger's mark.
From my testing they fixed the OP Colossus Slayer, before it would deal additional damage based on whatever the bonuses the player had, such as great weapon master giving +10 damage with heavy weapons also applying to the colossus slayer damage. I learned this cause I like playing strength-based rangers.
As far as duel wielding I can't answer any questions with that cause I never tried it.
