You can't twin hex
Twinned spell states (as of the first official errata):
To be eligible, a spell must be incapable of targeting more than one creature at the spell’s current level.
Hex is able to target more than one creature if the first one dies so it is ineligible. This targeting is referred to as "curse"ing but the effect is the same.
A typical spell requires you to pick one or more targets to be affected by the spell's magic.
The creature you are selecting to be hexed is the target of hex, so the new creature is another target of hex. The word target never appears in the text replaced by the action of cursing (emphasis mine):
You place a curse on a creature that you can see within range. [...]
If the target drops to 0 hit points before this spell ends, you can use a bonus action on a subsequent turn of yours to curse a new creature.
As such, the hex spell counts as targeting more than one creature. While at any given time only one creature is affected, multiple creatures may be targeted during the duration, so it can't be twinned.
This is reinforced in the Rules as Intended by Jeremy Crawford, lead designer, where he talks about the telekinesis spell on twitter:
Telekinesis can affect multiple creatures over the course of its duration. It's not eligible for Twinned Spell.
This is a comparable ruling because telekinesis, like hex, only targets one creature at a time, but can affect multiple creatures with one casting.
You can affect the same target round after round, or choose a new one at any time. [From telekinesis]
