Replace both HID headlights at once?

I always replace headlight bulbs in pairs since your final paragraph explains what I think is a good reason to do so, although I've never really though about extending the data set to other vehicles of the same age and mileage. I think within the same vehicle, one might say the lamps have been subjected to precisely the same road shocks, bumps and vibrations, something other vehicles might not share unless they've been driven over the same route at the same time! …an interesting question! …any Statisticians out there???

 

On the Prius, I would always replace both headlights at the same time, simply because it's rather inconvenient to change them, relatively speaking. I would especially do them both at the same time if you're paying the dealership to do it. It would be pretty sucky to have to double up on that labor charge in three months when your driver side bulb burns out.

On my vehicles with easy headlight access, I change as needed. There are times when I have gotten years more use out of one side than the other. I'm just not willing to take that gamble on the Prius.

 

The bumper cover does not need to be removed to access the headlights. There are many tutorials on this site and YouTube that can guide you. Order a good quality bulb - this is the most important step!
Although I have not changed them in pairs, I can see a good reason to do so if you do not have the time or a garage to do this now that winter is approaching and daylight is waning.
Phillips bulbs can be purchased for about 38.00 for 2.