A small follow-up to this answer: if you cast Witch Bolt + Twinned Spell, when you use your action to deal WB damage on your next turn, do you deal damage to both targets or to one? I lean towards the latter, following the answer above (that it's one spell with two effects/targets), but wanted to check.
You're targeting a second creature with the same spell. I think that for all intents and purposes all of the effects, even the use your action to damage them effect from Witch Bolt should continue.
If twinned spell is going to be possible it basically turns the target line of Witch Bolt into "you target 2 creatures" and then the action on following turns would damage both creatures.
One spell instance, two targets.