How does DC on a spell from two classes work?

I'm trying to understand how the same spell from two classes will work.

I know when memorizing spells you have tabs for the different classes. Are they kept separate that way when casting? Take for instance Hypnotism. With a bard the DC is charisma based and a Wiz DC would be intelligence based.

Are they kept totally separate? so that if I want to use the DC for Int I would learn it under the wizard tab and if I wanted the DC to be charisma I'd learn it under the bard tab? or are they combined based on which class has more levels kind of like how a TR chooses which class gets the perk?

Thanks for any info.

Not certain, but since they're from different spell lists with different stats to determine DC, its likely they'd count as different instances of the same spell & dc, spell pen etc. would be based 100% on the class whose list you take it from.

That's what I was kind of thinking, especially since they put in cost/casting time breaks for bards on certain spells. But assuming has gotten me into trouble in the past :)

Would be interesting if a splash of bard got you some price/casting breaks in some of the cc spells.

They're totally separate and independant.

A level 3 Bard/ 2 Wizard would cast Bard spells exactly like a level 3 Bard and Wizard spells as a level 2 Wizard. Your Bard spells would be CHA based and have a caster level of 3 (for duration, damage and spell pen). Your Wizard spells would be INT based and have a caster level of 2 (for duration, damage and spell pen). This applies even if you have the same spell on both lists.

The short answer is don't multi-class casters. They don't stack and are usually very, very, very bad.