I need to run at least 2 Pi together, one controlling hardware and another sensing its environment. I would like to keep weight down by using a single LiIon battery. I have one to connect to a PiJuice hat (12000Mah IIRC) which should power one Pi for a day. Is there a way that I can power both Pi from the single battery?
The best solution of your problem is using a High discharge Li-po battery cluster(12V) with a 5V-10A BEC. In this way you can get the power to run two pi's efficiently and without any throttle. But you need a powerful battery if you want to use it some more time without charging it now and then.
A single battery doesn't have that much discharge rate.
I'm assuming the battery is inserted in the PiJuice HAT on one Pi.
That means that Pi will have a 5V supply at the 5V power rail. You can use that to power the other Pi by connecting the grounds of both Pis and one or more of the 5V pins on both Pis.
The only question is would the 5V supply be enough to run the other Pi. As you haven't mentioned the Pi models being used we can't say.