User blog comment:Lord Loss/Discussion of the Week: Generation 2/@comment-5043481-20130211160446/@comment-4212769-20130212154025

Well, on the Dual Sword page (needs to be moved to Dual Blades) it is shown that it was introduced in MHG in Japan. What leaves an interesting conclusion:

Dual Blades are a first gen weapon in Japan but were first introduced in 2nd gen. (MHG came out in 2009)