I see, yeah. So once you get to the Golden Door, you can actually make your way to the Soldier holding the Golden Key without using any keys yourself. It does require an indirect approach, though.
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By splitting up again to use the NW Switch, you can then make a path with the debris and bridges to the central island, then use the switch there to get the other character over, too. From there, direct the debris again to make a path to the Soldier. Once you have the Golden Key, you can use it to exit the Soldier's room, then again on the Golden Door.