Front & rear, or ideally 360, is really what you want for documentation purposes. A rear-facing camera won't help you if you get right-hooked. And of course a front-facing camera won't help you if you get rear-ended.
Dual front and rear cameras are great, but they still leave a blind spot to the side. I once was riding solo on PCH, in the gutter, when a car approached me from behind, came up beside me, slowed down, and then the kid in the passenger seat leaned out his window and blew an air horn about a foot from my ear.
Even front and rear cameras wouldn't have caught that, or at least you would have seen the moments before and after. But not the act. A 360 camera would have caught it all.
Until then, yes, front and rear is pretty darn good and better than nothing.
Amateur recreational hobbyist cyclist
https://www.strava.com/athletes/337152 https://vimeo.com/user11846099