"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."
"You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class."
Brrr. That would be a drag. So then adding your spell casting levels from two mult classed spell casters only gives you higher level spell slots to allow you to upcast the lower level spells? Maybe this is what Larian was referring to when the talked about giving access to higher level spells than normal. One can hope.
Correct, as per 5e.
Larian's statements imply that they're implementing different rules, that [paraphrasing] "allow mutliclassed spellcasters access to higher level spells [fireball was specifically mentioned] earlier than normal." For more speculation on exactly how the above will be implemented, I'd suggest one of the other open threads about this topic.
Going back to your specific post: it would make sense if a multiclassed Wizard/Arcane Trickster *could* simply add their casting levels and use that to calculate spells known and prepared, as they both use basically the same spellset. Given ^ and Larian's stated desire to change multiclassing, I think it's likely that it'll work how your post describes.
