I'd assume yes, considering you have to offhand Aegis for it's trials, but maybe not!
Well it has to be equipped, what else would you do with it?
Alexander.Carrelo said:
»I would also assume that you can. I remember dual wielding two separate magian daggers for certain weather trials and getting progress on the offhand just the same as the mainhand. That being said, I agree with Dramatica's post above--assumptions and "should be's" don't always come through.
Obviously you would have to mainhand it for killshots, but the trial description is pretty clear about the conditions for the defeat trials... "target item must be equipped", offhand works for every trial I've ever done (made 6 daggers, 4 swords, and 2 katanas) and I can't imagine why Mandau would be treated any different than any others.