Generally, the heat metal spell is cast upon the metal weapons or armor of an enemy creature. But, assuming you had enough levels in sorcerer and either bard, druid or Forge Domain cleric, could you use Twinned Spell to target it onto the armor/weapons of two different creatures?
My suspicion is no, since Twinned Spell can only work on spells targeting creatures, not spells targeting objects.
Twinned Spell says:
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).
Heat metal targets an object:
Choose a manufactured metal object, such as a metal weapon or a suit of heavy or medium metal armor, that you can see within range.
So, unfortunately, RAW, Twinned Spell will not work with heat metal (as you suspected).
