If a CPU were to fail - which is extremely unlikely, per the other answers - there is basically nothing that the system could do to recover. Depending on the way it fails it could end up corrupting memory in strange ways, or destroying the process table, or who knows what else. If you were to have some sort of active monitoring system that keeps tabs on the CPU to make sure it's working well (and able to, say, roll back any changes made by the CPU during its death throes), that would also be another system that can fail, and determining software failure programmatically is pretty dang difficult (basically the only way you can practically do it is by having another CPU doing the exact same stuff at the exact same time and compare the results - which will then end up slowing things down such that there's no point to having another CPU to begin with).
That said, as rare as a CPU failure is, increasing the CPU count in a system will actually make your failure rate go up, as now you have twice as many things that can fail. You also have other subsystems that can fail as well, such as those which keep the CPUs' caches synchronized, and the increase in power consumption and thermal output also contribute to the factors behind overall system failure (and of course, active cooling fans are another point of failure).