I've successfully powered a Pi 3B+ using a cheap 12V 3A wall wart from aliexpress and a cheap buck converter from aliexpress. The 12V 3A provides for well above 3A when dialed down to 5.1V through the buck converter.
The most difficult part is soldering wire on to a USB plug.
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 ...
7805 can not give you sufficient amount of current. According to the datasheet it can give you maximum of 1.5A(You need a heat sink and cooler for that).
You have to use around 4 7805 IC for this operation, and also a filter system.
There is also some other risks, if you are powering your sensors or module from pi then there is a chance of over current ...
A raspberry pi need 5 volts to operate. So use a voltage regulator . It is best to use the official PSU though. But a 2 A power source is good. You can use the BEC(that is a very good idea). But keep the voltage regulation as low as possible(Assuming you are using an inverter)
The L293DNE is a chip and provides no voltage regulation. It requires two voltage INPUTS. One at 5V (Vcc1) for its internal logic and one at the motor voltage (Vcc2) you plan to use.
Of course you could power the Pi from the same source you plan to provide 5V to the chip.