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.
No, Twinned Spell only works with spells targeting creatures
Twinned Spell says:
Heat metal targets an object:
So, unfortunately, RAW, Twinned Spell will not work with heat metal (as you suspected).