Why change both batteries out at the same time? I change mine out when the hearing aid beeps. At roughly a dollar each, I like to use them until they are consumed. I keep a supply in my desk at work, a package at home, and some in the console of the car. I just swap the batteries out when needed.
Jeff
PS. My left ear requires more amplification than my left. The battery in the left aid is consumed more quickly than the right. There is usually about a day difference.