As a tester i have 2 X 3 12 V outlets available so i can also test the power supply that come with the cameras, but i have also a lot of USB ports at hand, though it is not often they see use.
As mentioned you can also get a USB charger with multiple ports and so run 2 cameras of that, the main thing is to get a good one and the ports have at least 2 A, of course then you also need to buy 2 USB wires to have between the USB charger and the cameras, and here if the distance are larger ( over 5 feet or so ) that you use high quality cables as there can be a voltage drop il lesser quality wires.
My 3 port 12 V splitters are hardwired into the fuse box, but most often they come with a plug so you can put it into the 1 socket on the dash or center console.
Hard wire into a fuse, leave your dash socket open for emergency things, so thats how it is in my car even if i have like 7 - 8 camera systems in the car at one time.
My entire glove box are my power grid, barely room for anything else in there when i have a lot of cameras in use.
Also a reminder, using different systems, the boot time ASo are probably not the same, so if you are going to mix footage from the 2 systems into one video, you need to sync the cameras up in post production, which are not hard, and can even be done automatic with the multicam feature of the editing software.
I will in that case advise you to use the sound from just the one of the systems and let that carry over into the footage from the other system, changes in the audio can be annoying.
