Is there or was there an aftermarket security system installed in the car? If the aftermarket system was connected directly to the door switches it may have created a common connection between the two door switches causing the lighting system module to interpret both doors as being open when only one door is open. If both lights come on whenever either door is opened, then I would bet a common connection of some sort. If both lights coming on only occurs for the driver's door and the passenger door only lights the passenger side, then I am at a loss.