I think they get a certain amount of miles/hours they eventually are prone to failure from vibration more than a new bulb, so one big jolt you may feel was no big deal can cause both bulbs to go out the same time. GL1800 uses the exact same bulb and lots of them go out the same time also according to guys on that forum.
That being said, I've never had both go out the same time. I replace one at a time, but usually the second one will fail whithin a couple thousand miles.
Guys recommend replace both the same time generally. I kind of think replacing one at a time works better, as if one is significantly older than the other, even a couple thousand miles, the chance of both failing the same time is much less.