I'm not really an expert on dual-wielding, but wouldn't a slower weapon with higher average damage (heirlooms being the first thing that springs to mind, but they wouldn't strike me as BiS ) for larger Threat of Thassarian hits, unless Thunderfury's proc has a high enough proc rate to make up for the damage lost on your Strikes. This list may be of use, although you probably need to filter it down furtherwhat is the best in slot weapon for a lvl 60 dk, excluding Thunderfury? Im talking about in the offhand position.
Because dual wielding is fun? :3Why would you dual wield?
Anyways, Shadowmourne.