Preservation of power when wiring batteries in series - battery life is reduced if load is kept the same
If you place two batteries in series but keep the load the same, the battery life is halved?

Battery life implies the number of battery cycles before the battery is no longer useful. &quot;Run-time&quot; would be a better description. Yes, the run-time would be halved. In practice it would be even worse because battery capacity decreases at higher discharge rates.

This should be a basic principle but I never see it stated this way, so I am wondering if I have missed something.

You have missed nothing other than your scenario would be unusual. There aren't many loads that can function properly over a 2:1 voltage range. Most products would be designed to work at one or the other. Exceptions include DC/DC converters but these will provide constant power to the loads so that as voltage increases the current drawn is reduced.