I'll try to upload an schematic ASAP (that means a couple of days), but the layout is very simple. In the mean time, think of a set of n DC switches wired to the Arduino inputs, a set of k outputs from the Arduino that control a board of relays/dimmers wired to 220V light circuits in accordance with the inputs. If a relay fails, no problem. One light circuit goes down but I still have lights in the house. If a swith fails, no problem neither. But, if the Arduino fails the whole system goes down having no connection between the DC switches and the AC 220V power lines. In that case I could go to the box where the Arduino is going to be housed and replace it with a new one, but that can take maybe hours to do (housing will be under a false ceiling and you need to get a ladder, open the access panels and so). I want something faster and easy to do. If I put two Arduino boards integrated in the system, having the same inputs, outputs and software, I can easily switch from one to the other with a manual override (as previously said) or on automatic. Anything doable in a matter of minutes will be fine.
No need for battery backup since if mains are down there's no need for light control. Power supply for the boards have to be independent, since the failure could come from the supply, too.