This popped up recently, confirmed to work:
BUG: people equip one handed and two handed weapons simultaneously! pls fix trion!!
If your inventory is full and you drag and drop an off-hand into your offhand slot, or drag and drop a 2h over existing items it stacks the weapons and allows you to equip things in combinations that you shouldn't be able to.
Not really useful for rogue/warrior, but ridiculous for mage/cleric. Hopefully Trion fixes it before it gets too out of hand.
Some people have used it to stack a 1h weapon, offhand, and staff.