Upload & Sell: On
usualy the best back up is a twin of your standard toy
That depends, as I wrote above, on what you are trying to accomplish with your "backup" camera. If it is a camera that you plan to use in tandem with your primary, then having a backup that is the same or very similar can help - though most photographers can adapt pretty quickly to slightly different interfaces if they actually use both cameras a lot.
However, there are other philosophies about what a backup/second camera is supposed to accomplish. For some, the idea is that you want two cameras that provide different strengths, with the roles of primary and secondary alternating depending upon what you shoot. Let's use the 5D2 plus 7D combination as an example. If you were shooting handheld and working with very active subjects, the 7D might be your primary with the 5D2 in reserve as backup. On the other hand, if you then had to shoot something with a static subject and were going to make large prints, the 5D2 might become your primary. Here the "backup" strategy is also a strategy of expanding the range of work you are equipped to do.
Another approach regards the "backup" camera as one that will almost never be used - simply there in case the unthinkable happens. It mainly is only for emergency use, and it would allow you to complete the task in suitable fashion should the primary fail.