First question I have is what are your expectations for this application? What's the intended use?
Just from previous experience.. any aftermarket camera setup is kind of worthless compared to a factory setup.. it's cool to be able to see stuff, but depending on the viewing angle and position of the camera it's hard to rely on to, for instance, back up or pull up right up against something.
If you only have 1 camera input, that means you won't be able to set independent guide-lines on the screen overlay, which means you'd be relying on the look and feel for distance vs where you bumper actually is.
With that out of the way, nothing is impossible.. I'm not electric engineer, but even with a single camera input it's possible to feed in more than 1 camera. You can either wire all this stuff yourself, or just buy a dedicated chinese camera splitter from amazon (something like this
Amazon.com: Anytime Backup Camera, Video Switcher, Intelligent Multi-Function 4-Way Video Switcher (4 in, 1 Out) for Cars, Trucks, RVs : Electronics)
That may be overkill for what you need... ultimately the way the headunit/backup camera works is when you throw your car in reverse, a voltage signal is sent to power on the camera, once it's powered on, the signal from the camera triggers the headunit to overlay the camera feed.
This means that really all you have to do in order to have any of the cameras show up on your screen is power them on to have the signal sent. The complication with a single input is that you need to make sure you're only powering 1 camera at a time. More than likely what you'll want is an on-demand switch to kick on the front camera, which also simultaneously cuts off the power (or signal) circuit from the rear camera (meaning that even if you're in reverse and your rear camera is automatically on, hitting the button will kill that power and switch it over to the front camera).
I'm pretty sure all of this can be done via a simple toggle switch and relay setup, but a diagram is not something I can readily provide.. now that I got to the bottom of typing all this out I'm thinking maybe that controller/splitter I linked maybe the best option if your wiring knowledge is limited..