It is possible for me to equip a 2-handed weapon in the primary weapon slot and a 1-handed weapon in the secondary slot at the same time. Here is how to reproduce:
1. Equip 2 1H weapons, one in each slot.
2. Drag the primary-hand weapon from your inventory and drop it on a 1H weapon in your bag.
The 2H weapon equips in the primary weapon slot, but the secondary-slot 1H weapon does not unequip automatically, as it does if you use other methods to equip the 2H weapon.