Seems like a common problem. It happened to my dad's Innova 2.5V sa driver's side. I also remember seeing a thread about it several months ago.

I would think that they never went out at the same time. But that seems to indicate a design flaw with the contacts.... tolerances, contact pressure, or even the chemical makeup that might cause the bulb and the socket contacts to react and corrode at the point of contact.

I haven't encountered it personally as I never had the conventional bulbs long enough. A few months after buying my Innova, I switched them out with a pair of 1157 red LEDs.