I'm considering playing an Unearthed Arcana stone sorcerer.
After I hit with an attack and choose to use a smite spell, can I then use twin spell to double cast that smite spell?
If you look through the list of smite spells that you get, they all have a range of "self". The text of twinned spell (PHB 102) explicitly prohibits spells with a range of self (emphasis added):
When you cast a spell that targets only one creature and doesn’t have a range of self, you can spend a number of sorcery points equal to the spell’s level to target a second creature in range with the same spell (1 sorcery point if the spell is a cantrip).