You don't need Quickened Spell to cast 2 leveled spells in the same turn, in this game. I've cast Misty Step and then Magic Missile with a Wizard, more times then I can count. And I've cast Healing Word + Cure Light Wounds in the same turn as well. Any spell that can normally cast with a bonus action can be followed or preceded by another leveled spell. Now I know it's not allowed in 5e, but it's been allowed in BG3 since the beginning.
Yes, but the difference here is that normal BA spells are pretty weak. Healing Word, Misty Step, Sanctuary, etc. But since quicken can make any spell cost a BA, it allows for incredibly powerful turns. As in the example of fireball+fireball. A sorcerer could wipe out many enemies before they or any other character even gets a turn.
Edit: see the example given in schpas's comment above
