Happened to me too rob. I'm not using any cheats. The client/server permitted it. Duel wield any 1-handed weapons(2 of the same for me) and then wield a 2-hander. It stuffed it into 1 hand by default. Also i found some odd sorting or resorting of the weapons in question when i tried to change them. It was as if the primary and secondary slots swapped position in inventory. You might want to have another look. I use IE11 and Chrome (if that helps).
I found it odd but i thought it was a known mechanic. 3 days into the game i wasn't going to question things til i knew what i was talking about.
Anything else u want to know just ask. Happy to help stomp out glitches and hacks.