The neck selector switch is designed to be centre off, as all the controls are tied to the selected neck and can't be shared. You will need to use a different layout if you want both necks on at the same time.
Are you getting any adjustment with both volumes when in the middle pickup selector position, and is this still with just the tap test? If wired as per the diagram it should work just like an LP, as in you can adjust the volumes to balance the sound but turning either volume to zero will mute all sound. Can you post some pics of the cavity, if you can get some showing your connections to the pots.