No you don't. Double Saber Caster can give 2 Servants 80% gauge each. Shiki (Assassin), Chiyome, Carmilla, Jeanne Alter (Berserker), Vlad, Kintoki, and Kama all have battery skills that are 20% or better.
Your refund issue is only a problem for Shiki (Saber) if she instant kills both enemies. Jason, Tomoe (Saber), Nero, Saber, and Mordred don't have to worry about that. Or, if you're using Shiki (Saber), the only Shiki (Assassin) option you can't replace her with is Jeanne Alter (Berserker) in worst case scenario of instant killing both enemies.