Twinned Spell says:
When you cast a spell that targets only one creature and doesn't have a range of self […] To be eligible, a spell must be incapable of targeting more than one creature at the spell’s current level.
Does hex meet this definition?
While hex initially targets only one creature, later on it explicitly allows you to choose a new target which could mean that it would be targeting more than one creature. And Twinned Spell does not seem to limit the restriction on targeting to any given time frame.
You place a curse on a creature 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.
Does this ability to choose a new target disqualify hex from being used with Twinned Spell?
Best Answer
You can't twin hex
Twinned spell states (as of the first official errata):
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.
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):
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:
This is a comparable ruling because telekinesis, like hex, only targets one creature at a time, but can affect multiple creatures with one casting.