Try Rich P's suggestion first. I've had it happen more than once. You don't always notice how dim your lows are getting (one light) until the second goes out. If that isn't it, check the connector at the bulkhead near the airbox for corrosion, a common problem on pre-97's. When you replace the first light, also take note of the condition of the connector there, and replace it if it's too corroded. Clean it up and lube it with vaseline or dielectric grease anyway.