It's all to do with failure rates, lamps have a well defined lifetime 20,000 hours or whatever. As they were both manufactured and operated together, it is reasonable to expect them to fail within a similar timeframe. I'd recommend either changing one lamp after 6 months or so, or carrying a spare lamp kit.
I am amazed that BMW haven't fitted either xenon or LED main lamps to the RT as standard. The competition is way ahead of them in this technology.
Ted