@mangax going by the schematic you would have to modify the port to even accept a dual controller to a single port and then the firmware would most likely not read those as separate controllers.
[image|1454093]
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=== Update (05/15/2018) ===
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Neither one of those options will allow you to use two controllers on one port. The only way to do it with that setup is by using both controllers in separate port and the switching from te controller to the light gun. so something like controller one in port one, controller two in port two, switch or diode in between controller two and light gun. That way you should be able to switch from controller to light gun, but not both at the same time
@mangax going by the schematic you would have to modify the port to even accept a dual controller to a single port and then the firmware would most likely not read those as separate controllers.
[image|1454093]