Multiclass and Spells - how does it work?

[krugaan]

I thought it was clear. You can prepare 9th level
Spells but you can't know any, so you can only upcast existing spells to 9th level if you want to use those slots.

er, the other way around, you mean? Clerics and druids always know all their spells, but can only prepare ones that their available spell slots allow.

Regardless, it's ridiculous to assume it works the other way, because then the best sorc builds will all be 18 sorc / 1 cleric / 1 druid or 17 sorc / 2 lock / 1 cleric or druid.