I know the Elf/Orc Blood traits make the Half-Races vulnerable to anything that affects full-blooded Elves and Orcs, so I'm guessing Favored Enemy Humanoid (Orc/Elf) is going to do the trick, but does Favored Enemy Humanoid (Human) work against Half-Elves/Orcs?
No, Favored Enemy (Human) won't help. Half-elves have the Elf subtype, and half-orcs have the Orc subtype (that is basically what the XXX Blood abilities of these two races means), and the Favored Enemy ability works by subtype.
None of them has the Human subtype.
I should've just read the blood ability rather than being distracted by the random subtype thing.
SRD:
—Elven Blood: For all effects related to race, a half-elf is considered an elf. Half-elves, for example, are just as vulnerable to special effects that affect elves as their elf ancestors are, and they can use magic items that are only usable by elves.
Looks like no matter what little mister half elf is going to never show up as human. Pretty odd that. Seems more interesting if they would get both bonuses and penalties, but only half as much.
Solve the problem with Favored Enemy (half-elf) and Favored Enemy (half-orc) and say that a half-orc is not an orc and not a human (ditto with half-elf).