User blog comment:FairyRaven/Fairy Tail Chapter 401: Igneel vs. Acnologia, Review and Commentary/@comment-24467372-20140922162609/@comment-7418318-20140922200628

@ML: You don't need to list off how most people's strong techniques are two-handed, I know that too, BUT!

"We also saw Laxus's attacks be much more devastating with two hands" -> Jura might disagree with this :P

Apart from that, just because it's like that with most people, doesn't mean it's the same for everyone or this one in particular. Gildarts is totally overpowered with just one hand and nobody made a mention of him declining in power from losing one arm for example.

Or Jellal: He drew Abyss Break with only his right hand. Now we don't know how that compares to Sema, but considering how both Erza and Jellal thought AB would destroy the entire tower, it almost asuredly blows Altearis away, which also needed both hands. That would support the argument that attack-power is independent of how many hands you use.