this is just a Visual bug, When i press "F" to swap main-hand with off-hand while i have CAD in hand, it appears to be in both hands

It will also do it if you swap it with an item in your hotbar.
example you have the cad in hotbar position 2 and you swap it with a sword in position 4 with a keyboard shortcut it wont update the textures until you click on the sword