I'm going by what the equipment chart and wiki page states that the Flamberge has no secondary option to halfsword. That sounds strangely out of place given it's very design encourages that, yet other swords in the mod have that option where it does not.
Given that i'm new to this mod, if the flamberge in fact has the option for halfswording and the information just hasn't been updated I apologize for cluttering up the forum with this request, but if it doesn't have the option, it REALLY should.