I am not big on using 2 conventional systems, with the big main units front and rear and then putting the remote "rear" cameras on the sides.
I did do a conventional system front and rear, but for side cameras i when with a dual remote system, mainly the K2S as its remote cameras are much smaller / about half the size of any other rear camera i have seen.
The K2S camera beside a conventional mini wedge rear camera as used by viofo - street guardian and many others.
Mind you the K2S have since died on me, so ATM i have no side cameras, but i did get +2 years out of the system.
Side cameras on my car.
Of course using 2 similar systems, that is a plus in the way that they will have the same boot time ASO, ans so files from all 4 side should be in sync, strait off the memory card.
Mind you most video editing softwares do support some form of auto sync between several cameras, this are often a audio thing, but if you dont like to mess with such things well then 2 similar systems are nice.