I’m not actually attempting to do that at the moment. Having the spare battery and external charger was more just for exploring possibilities, and perhaps for occasional circumstances where I really need to double the available time.
Right now I am living with the amount of time on one battery and just putting it on charge once it tells me to.
However my forecast answer to your question would be “no” - if the meaning of your question is: if I had to spend 24 hours off grid would two batteries be enough? However see next.
It’s touch and go. Charging the battery externally takes very close to the same amount of time that it takes for the internal battery to go flat (with the obvious variable that the latter depends on how much I have been doing with the phone and what components are hardware killed and what is connected to the USB-C port).
So if the meaning of your question is: given constant availability of a grid supply could the phone remain available forever with two batteries but without ever putting the phone on charge? then the answer is a “touch and go yes”.
To refine that answer a little: external charging takes about 8h 20m and I will usually put the phone on charge when the battery depletes to 20% which may be sooner than 8h 20m, but if I pushed my luck with the internal battery and let it go closer to 0% then I might be able to go long enough to change the battery after the external one is charged and before the internal one is flat.
Really if I wanted that workflow then I would need to get a better charger (higher current). The one I bought was cheap and just for exploring.
Also, the fundamental problem with this interpretation of the question is that, well, given constant availability of a grid supply, I can just put the phone on charge. I don’t even need a second battery.
There would be a carefully arranged circumstance where you can argue that this situation makes sense i.e. where you are coming in and going out in a complicated dance, while the spare battery is on charge somewhere and you can rush back at the right time to swap batteries. But that isn’t how I want to live my life - hence why I am keen for Purism to implement a sleep mode that is good enough to go 24 hours on one battery (preferably more than 24 hours).
A third interpretation of the question is: getting through the day means getting through, say, a 16 hour period off grid. Then the answer is obviously yes provided that the phone is only subject to “regular usage”. (If I have a peripheral sucking power at the USB-C port e.g. USB-C portable drive, then all bets are likely off.)