We are all aware of the need to have two cameras when working, certainly as a professional that is part and parcel of deserving such a tag. But even as an enthusiast, if those shots are never to be taken again, such as traveling to exotic locations or when a guest at say a wedding, it is not a bad idea either. In the case of the non pro, often a competent compact is decided upon as the spare.
Even allowing for todays reliability, things do go wrong at some stage in a picture taking lifetime, no matter your choice of brand, not overlooking the occasional accident. But how do you choose and use your spare camera? Nothing is cast in stone, but here are some real world considerations. To keep this as brief as I can I will talk in terms as a working photographer.
When you start out it can all come down to the cheapest back-up and or the lightest option. But when there is more of a choice, the common thing is to select a model that complements the main camera. Maybe that will have different features and be on a later design? That is often chosen to increase the range of options or have improved performance of this or that feature.
However, in my experience that is all fine and dandy until the dreaded moment comes and you have to use it. All of a sudden there is a ‘new’ and different camera to get to grips with while people and or lighting need dealing with and organising. Ergonomics, menus, icons, size and weight are all different. Nothing wrong exactly, but you have to get into a different mind-set and quickly.
So my first suggestion is to make sure you are as familiar with the back-up body as you are the first. Otherwise you may miss shots by having to stop and think too much when it is needed. Your camera (any of them) should be seamless in its use, and so does not get in the way of picture creation. That takes practice so use your back-up more than just when you really need it.
I prefer to work with two of the same type of camera or very similar when I can. Any transition is then relatively quick and painless. So say a Nikon D300 and D300s, D3 and D3s or a Canon EOS 5D and 5D Mk II as popular pro camera examples.
Batteries, chargers and storage cards are shared, so less to carry, while the cameras format does not require a shift in approach or a different lens to adjust for a now different angle of view, due to a larger or smaller sensor size. I also swap cameras to share out a days work, just in case.
That said, (and I hope Im not tempting fate), I can not recall the last time I had a camera stop working on me, but in case you think Im just lucky, too make you feel a little better, I did drop an SB-900 recently causing some damage to its wide-angle diffuser. I await the repair bill.