Never.
Sure if you are strong enough you can hold one in each hand but... Try fighting effectively with them. You can't.
See.... that's exactly why I think it should be possible.
To me, the way to deal with things like that is not to just arbitrarily say, "No - you can't do it," but to make it so that you can do it, but make it realistic, which is to say, obviously ineffective. The weapon would be too heavy and long to handle well with one hand, it wouldn't do much more damage, if any, with one hand (and might even end up doing less than a blade made for one hand), you'd be defenseless as soon as something got inside of your reach, it'd burn through stamina very quickly, and I'm sure there are more problems I'm not thinking of right at the moment.... so fine. If somebody wants to swing a heavy weapon slowly and poorly and do less damage - go for it. At least his opponents will get a good laugh out of it before they kill him.