Which Ability do you use for multiclass casters sharing spells?
Ex: Artificer (Int) Sorcerer (Cha) share the Catapult spell
What ability do you use when casting it for the attack roll and save DC?
You use the spell attack bonus or spell DC from the class that granted the spell. Sure, both Artificer and Sorcerer grant access the the catapult spell, but the player is only going to gain the spell from a single source.
From the Multiclass Spellcasting rules (PHB, pg. 164):
Each spell you know and prepare is associated with one of your classes, and you use the spellcasting ability of that class when you cast the spell.