The two batteries inside the Duo will allow longer usage.
from the Duo page:
- Up to 31 hours video playback when using the inner display
- Up to 44 hours video playback when using the outer display
I was a bit disappointed in these numbers because, despite having two batteries and the outer display being smaller, the Duo still has 1 less hour of video playback on the outer display than the iPhone 18 Pro Max.
You can run one iphone at a time and lap the duo. 64gb iphone SE is going for $140 at best buy right now. I could tape up 10 of them into a jacobs ladder then I'd have 640gb of storage and maybe 10x the battery life of the duo, and money to spare for future expansion iphones.