Threw casting spells via different means, you have learned to cast spells via different methods. Pick two spell casting modifiers that you can already use from different spells , when you cast a spell that uses either of those modifiers you can use the other modifier. You can use this feat a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.

Example: You can cast Acid Splash with Intelligence and Eldritch Blast with Charisma. You can now cast Acid Splash, or any other Intelligence spells you can cast, with Charisma and you can cast Eldritch Blast, or any other Charisma spells you can cast, with Intelligence.