Should runners be allowed to wear both headphones on a trail?

It would be tough to police and a waste of resources to have someone out there solely for that purpose. Bone conduction headphones are becoming more and more popular (e.g., Aftershokz) partially for that reason - to allow runners and other athletes to be able to better hear what's going on around them, while listening to music or podcasts.

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Perhaps an air horn on your bike would better get their attention. 

Instead of complaining, why not make money? It's your idea, so invent an app that you market to runners for their safety that picks up the sound of a bike, a bike bell, or someone saying "on your left" approaching from behind and alerts the runner in their left ear. I mean, it is people with headphones plugged into iPhones or smart phones, which also have microphones on them, so it would be an easy enough app to make. Even if you don't have the coding ability, you patent the idea and then get someone who knows how to code to code it for you.

You see an unmet need. Don't complain. Be glad. Invent. Profit. 

Or don't and stay poor and grumpy.