multiclassing spellcasting modifier?

[FoxxyGramps]

so lets say you start off as a warlock for a few levels, then go into wizard for a few levels. what would your spellcasting modifier be? charisma or intelligence or both? also, would you be able to cast warlock spells with the spell slots your wizard grants?

[bid]

so lets say you start off as a warlock for a few levels, then go into wizard for a few levels. what would your spellcasting modifier be? charisma or intelligence or both? also, would you be able to cast warlock spells with the spell slots your wizard grants?

If you are a warlock 3 / wizard 5, you are both separately.

When you can a wizard spell, you do so as a wizard 5 (using Int DC).
When you can a warlock spell, you do so as a warlock 3 (using Cha DC).

You can use either slots for any spells, and as detailed in the MC section, you always use the proficiency bonus of a level 8 character.