BUT if you had an attack power that said
HIT: 2d6+STR radiant damage
AND you had a different power that said
TRIGGER: When you deal radiant damage
EFFECT: target saves or takes +CHA fire damage
that would be two different damage instances, of two different damage types, from two different damage sources because it's two different powers--any radiant damage power could have triggered the save-or-fire damage power.