Oh I am also a proponent of 2 saving throws to get the "Sit Down. You're Done" level of effects.Here is the thing: I don't actually see any value in players not being able to target a weak save. It just means that by random chance, sometimes a player character can't contribute with their useful spells. Is that really great? Or alternatively, it means a spell casters entire effectiveness hinges on them optimizing their spell load-out for this one purpose, needing more system mastery and reducing built options.
It's far better if you design the game so that targeting weak saves is feasible but a single save doesn't turn a challenging fight into a cake-walk. It should be a serious of smart decisions that yield victory, not a single decision followed by a single roll. And especially great if the decision is based on stuff happening during gameplay (either advance research or learning the monster's weakness during combat), not as character building exercise.
Fail first Charisma saving throw and you start fading away.
Fail second Charisma saving throw and...