If all of your bags are full and you drag and drop a 2h weapon on top of your existing single handed items it will stack with them. This works with swords and shields as well as totems and such. You must use the bank to hold it for the drag and drop, of course.
alternate method...
First find your offhand item while wearing a 2-hand. Next go to the bank and put the offhand item in the bank. After that fill up your inventory. When your inventory is full, drag/drop the offhand from the bank to the equip slot. It will now equip, and since your inventory is full, it can't unequip itself. Rinse and repeat with the mainhand... makes for some INSANE spellpower on my mage. Gonna miss this one when they fix it.
you can wear a 1-hander, 2-hander, and an offhand item all at once! Four hands! Once complete you can empty your bags without losing the glitch.
This bug will be fixed within day or several hours