1) Wear a 2-hander
2) Have your current bag full, except for one empty space.
3) Have someone trade you two items, the second of which is the 2 hander you want to "equip".
4) That's it. First item will go in your bag, second will be "equipped".

Fisr of all you have to to turn on the auto equip option from your options-> gameplay-> interface and there must not be any offhand item at the slot.
Working at Demon hunter and Monk. Not personally tested at other chars except wizard that it's not working