User blog comment:EvanDude345rv5/Fairy Tail 333 Review : Ultear FTW !/@comment-5580383-20130603183306/@comment-7418318-20130603184926

In the Sasori fight she wasn't exhausted from a single attack, but from constantly dodging for several minutes (as baba-sama pointed out, enhancing her muscles with chakra, making her more effective, but also exhausting her more than "normal" movement) followed by a strong attack.

As for Kagura, she later made two-handed attacks that didn't erupt through half the city, so I'd say it doesn't matter much wether you hold the sword with both hands or just one, when you make a magical attack.