I don't really know DnD's rules about that but being able to cast 2 level 1+ spell with metamagic looks really OP.
I can't imagine how powerfull the sorcerer will be at higher level compared to any other caster.
I think it would be better balanced if you could only use 1 spell + 1 cantrip (or eventually level 1 spell) per turn with this metamagic option.
I played real DND last night. Our Fighter is an Eldritch knight meaning he can cast Wizard spells (kinda like the Arcane Trickster in Bg3). He cast the fireball spell, action surge, and cast the fireball spell again. He killed over 60 enemies (it was a very large siege fight). As a fighter, he can only ever do this once. At around 8th or 9th level a sorcerer (without the BA limitation) can cast 4 fireballs in two turns, OR cast 2 upcasted fireballs in a single turn. It gets insane very quickly. How about multiple upcasted Magic missiles? You could put 12-20 darts in one target. Assuming they deal 3 damage each that’s 36-60 damage each turn.
I think they should nerf every class to not allow a BA spell and an action spell. The enemies can do what you do as well. Imagine trying to kill a boss that has or is a cleric. BA healing word cast at 2nd level, and cure wounds at 2nd level. It’s broken IMO.
