That's just the animations combining as they are supposed to. That isn't a glitch.
However, I do hope that they will include dual wielding. It is more relevant for wielding pistols; bucanneers in the caribbean would often carry multiple pistols in bandoliers, loaded just before battle so that they could get off more shots before having to reload even one of their weapons. That led to faster responses to enemy fire and more efficiency.
The same principle works IRL. If you have four fully loaded guns and are holding two at once, when one of them jams or runs out of ammo, you can use the secondary gun to make a snap shot reaction. You will be less accurate, but your rate of fire will be ever so slightly more reliable, which can be crucial.
However, I can imagine that compared to Skyrim, dual wielding isn't quite as relevant. There's no magic, no shields (actually, I'd love a ballistic shield, wouldn't that just make sense?). You are far more reliant on your VATS and guns, which makes most uses of dual wielding obsolete. Although, having a secondary weapon might increase your VATS special options, so there's an idea.