Both of the damage bonuses are multiplicative, so if you're using a bow from high ground then having a mix of Ranged and Huntsman will result in more damage than all of 1 and none of the other.
However, because Huntsman has a high ground requirement, no crit chance, and has a default 20% bonus with 0 investment, it's not as appealing as Ranged.
In fact, a character has to have 5 points in Ranged before 1 point of Huntsman is more damage, and then continue at an even pace afterwards (this is due to the default 20% high-ground bonus).
Also consider that skills are capped at 10, and of course Ranged is only viable to Bow/Crossbow while Huntsman works for any high-ground attack, (great skill for multi-element mages looking to stack damage).
A few examples: (not considering crit, which is minor anyway)
10 points in Ranged is 150%. With high ground that's 150%*1.2, so 180% damage.
10 points in Huntsman is 170% with high ground.
5/5 split is 100%*1.25*1.45 = 181.25% with high ground.
7/3 Ranged/Huntsman split is 100%*1.35*1.35 = 182.25% with high ground.
10 points of both results in 100% base, 50% Ranged bonus, and 70% High Ground (50% Huntsman and 20% default). That's 100%*1.5*1.7 = 255% damage.
So as you can see, Ranged is basically always better until you cap it out, assuming your weapon applies. The minor damage bonus possible from mixing and matching isn't worth the loss on the times high ground isn't available.
