One is wired to the reverse lights, so when I select reverse, it powers the camera, and the head unit sees the camera and automatically switchs to it. The 2nd camera is hooked to accesory power so that it is on whenever the car is going (and I can select it from the inputs on the head unit)
On my last car I had a switch to accessory power, and when I wanted the camera I just clicked the switch on, and the display would automatically switch to the camera (It was on a Large screen GPS's 2nd input)