So, you mean that if you multiclass 1 level into wizard, that youcould only use those learned spell with the wizard spell slots, right?
Yes exactly. you can cast the Wizard spells you learned, and you can learn any level spell you can cast as a Sorcerer - so you could have 2 levels in Wizard, learn haste from a scroll (level 3) and then you can cast that with a Sorcerer level 3 spell slot AND you can Twin the spell.
Totally broken.
Hmm, i wonder if that´s on purpose or actually a bug, because with any other class you can use you current level spell slots, wich seems to be kind of shared between classes, with any of the spells of the classes you are multiclassing into. For example, a level 4 cleric/level 1 druid can use the higher level spell slots to cast druid level 1 spells upcasted.
There are 3 parts of this that are broken though.
1) A Wizard can only learn spells they can cast as a wizard - so a level 1-2 wizard would be limited to level 1 spells that they can learn.
2) You can't cast a Wizard spell with a Sorcerer spell slot - this is the only part you have contested.
3) You shouldn't be able to apply a metamagic feat to a Wizard spell that you learned.
So assuming you are right about 2, the first condition here should prevent you from learning spells above your wizard level - as it does when you are a pure Wizard. A level 1 Wizard cannot learn Level 2 spells. So we have a bug/exploit.
Oh and there is an additional Exploit that lets you keep Wizard spells you learned by leveling when you respec. Respeccing should wipe those spells as if you had not chosen them. In the case of - "well it's a spellbook" - this would be a prime example of why this is an exploit, because Sorcerers don't cast spells from spellbooks.
For reference:
PHB Pg 114, 2nd paragraph
“Copying a Spell into the Book. When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a spell level you can prepare and if you can spare the time to decipher and copy it.”
PHB pg 164, spellcasting, 2nd paragraph
“Spells Known and Prepared. You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. If you are a ranger 4/wizard 3, for example, you know three 1 st-level ranger spells based on your levels in the ranger class. As a 3rd-level wizard, you know three wizard cantrips, and your spellbook contains ten wizard spells, two of which (the two you gained when you reached 3rd level as a wizard) can
be 2nd-level spells. If your Intelligence is 16, you can prepare six wizard spells from your spellbook.
