Same thing here. I had a conversation with one of my buddies about this one day, he mentioned that when one burns out it puts a slight overload on the existing bulb, decreasing its life. How true it is I don't know, but I know I've had two bulbs burn out within hours of each other. Seems to be a recurring problem on my mom's VW...

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

Update on the simultaneous bulb burnout...

I looked at the outside video tape (work video, that is) from the morning of the day I recognized the problem.

I drive 40 miles one way, I leave the house with the headlights on since it is usually around 6:15 am. When I pulled into the parking lot at work, the video shows parking lights on ONLY... That was at 7:00 AM

So between 6:40 AM and 7:00 AM, both low beams burned out. That's simultaneous enough for me.....

So glad I used to watch Columbo detective show when I was a kid!

anyway, I checked a new bulb at the store the same way I checked the bad drivers side one and had continuity "beep" any way I tested the new bulb but not with the old one....

two new lowbeams are in and problem is solved...

Id imagine if two high quality (consistent manufacturing) bulbs were installed at the same time, subjected to the exact same voltage their entire life and the same jarring motions they would burn out at a reasonably close time. If their not burnt out I would check the headlight switch. It may use the same contact for both circuits.

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1981 300SD 512k OM603

If they are true "EURO" lights they are NOT legal for inspection, Whether or not they are noticed is a different story.

Look to see if "DOT" is present on the glass markings.

If it does not appear, it is NOT legal

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

for customer.

.

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ASE Master Mechanic
https://whunter.carrd.co/

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
2003 Volvo V70

https://www.boldegoist.com/

Here's some ideas and tips that come to mind:

A power surge can knock out more than one bulb on a common circuit.

Check out voltage regulation on the car.

Solid ground connection can be as important as solid "hot" side connection, poor on either side can cause weird voltage fluctuations.

Check the bulb circuit (voltage) with a bulb installed and again with no bulb. If you get a strong reading with no-bulb, but then with a good bulb installed you get no light... you've got a poor continuity / weak connection on either ground or hot.

Determine if probably fault is your headlight switch by by-passing it, with car off, use a heavy gauge jumper wire directly from battery to the connector on the headlight.

A crude quick test, if you're on the road with no test gear... swap bulbs left to right to see if the problem follows. Swap left and rigth fuses, ditto.

My car came with with a carefully packed box of spares for the more critical lamps, found it in the trunk. Thanks PO! Not a bad idea to keep some spares on the road, they're lightweight.

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Cheers!
Scott McPhee

1987 300D

When I drove cross-country with my now-ex, both headlights on her VW burned out at about the same time. I know this because I did a very thorough pre-trip inspection the day we left, including a full light check. On the first gen New Beetle the low beams are run as DRLs, and they used crap bulbs. Over the course of a day, both bulbs burned out and left me with nothing once the night driving started.
FWIW, I don't think it's possible to change those Beetle headlights without breaking the housing.