I'm not sure I understand completely, but I'll give it a shot...
It almost seems like maybe it's set up to provide a 360* light at anchor and only 135* rearward facing light when used in the running position?
You could wire the rear bulb only to come on with the NAV switch, and wire both bulbs to the ANCHOR switch. You will want to be acrefull how you wire it, or it may send voltage through the NAV side when the ANCHOR switch is on, illuminating all the lights.
Does this make any Sense?
Have any Pics?