To quote the Player's Handbook page 54 under the Bard Features:
Magical Secrets
By 10th level, you have plundered magical knowledge from a wide spectrum of disciplines. Choose two spells from any class, including this one. A spell you choose must be of a level you can cast, as shown on the Bard table, or a cantrip.
So that is the relevant portion. You can cast 9th level spells, so you can learn a 9th level spell from any class.
Also, I should clear this up. You have 9th level spell slots, and you are a Bard and a Wizard. You can use those spell slots for either classes spells. And when you level up and get new spells, they must be "of a level for which you have spell slots." And we both know already that you get spell slots based on the total levels of all your spellcasting classes. Thus, whenever you level up in a spellcasting class, you can learn spells for whatever spell level you have slots for, even if you just got your first level of Sorcerer and it's your 11th level. Then you can get 6th level Sorcerer spells, or spells of a lower level if you so desire.