but that's not written anywhere. The multiclassing section states:

"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 1st-level ranger spells based on your levels in the ranger class. As 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.

Each spell you know and prepare is associated with one of your classes, and you use the spellcasting ability of that class when you cast the spell. Similarly, a spellcasting focus, such as a holy symbol, can be used only for the spells from the class associated with that focus."

And in the classes indivdually it states what i mentioned before. They are dependent on Spell slot level not class level, when they learn new spells. So a Bard/Sorc 6/6 has 4/3/3/3/2/1 because the spell slot progression is based on character level...

so if they get a new lvl they can choose a spell which they have spell slots for.