Maw How much each stat contributes to damage per spell line (INT vs CHA split) is confirmed to stack but not yet quantified.
Confirmed that both do affect damage, but the stats are on diferent scales. Swapping gear to lose -39 cha and add +36 int, for instance, had this effect:
Anguish: 750 -> 839 dmg
Mindshackle: 119/tick -> 134/tick (without the 50% bonus in both cases)
So int is substantially more valuable than cha for damage.... in addition to the extra mana / regen.