This might be the wrong forum for this post, I am a bit unsure, so if any mods see this thread and think it should be in a different section please move it, as I would appreciate getting the appropriate help with this question.
That being said, I seem to recall a feat or maybe an archetype that allowed a wizard to memorize two spells in a single spell slot and when one was used they both go. So for example: I take the feat and memorize fireball/lighting bolt both in the same 3rd level spell slot. Once I cast either of them both are gone for the day because they occupied the same slot.
I do believe this was a Paizo feat/ability somewhere, but I might be thinking of a third party one. If anyone has a clue to what I'm talking about, whether its Paizo or third party, I would very much appreciate being pointed in the right direction here.