Bards can use a musical instrument as a spellcasting focus for bard spells.
If I have levels in both the Bard and Sorcerer classes, a musical instrument to use as a spellcasting focus, no access to an arcane focus, and a known Sorcerer spell that also appears on the Bard spell list but needs a material component (such as Invisibility), can I cast the Sorcerer spell as if it was a Bard spell? Since it is a Bard spell, I know it, and I have a spellcasting focus for it.
