The Twinned Spell effect works on spells that target a single person, so would it work with Chain Lightning, which only hits a single target in the start, but slowly spreads?
Twinned Spell says:
When you cast a spell that targets only one creature [...]
With the release of the errata for the PHB, we have this additional restriction:
To be eligible for Twinned Spell, a spell must be incapable of targeting more than one creature at the spell’s current level.
And Chain Lightning says:
You create a bolt of lightning that arcs toward a target of your choice that you can see within range. Three bolts then leap from that target to as many as three other targets.
So no, you can't use Twinned Spell on Chain Lightning. Chain Lightning targets up to four creatures, and Twinned Spell requires the spell to only be capable of targeting one.
If a further argument is required, Jeremy Crawford says that Twinned Spell cannot be used on Chain Lightning.