This would be a purely cosmetic option, no change to game mechanics.
Basically, your sword or gun would have a "dual wield" accessory slot, and there would be a matching "dual wield" accessory ticket found in lockboxes that you could take to Bechamel to make your weapon into a dual-wielded one.
When equipped, a duplicate of your sword or gun would appear in your left hand, much like how the Acheron's scabbard is held, not interfering with the shield. The attack animation would be modified so that attacks alternate between the two weapons, but the attack speed and combo mechanics would be unchanged. The charge attack animation would also be modified appropriately to look twice as badass.
Just want to have a chance to look as awesome as Vaelyn...
NOOOOO!!! No dual wielding....wait...hold the phone... OMG COOKIES AND RAINBOWS THIS IS AWESOME