By paying twice as much you could get four times the performance. This is no longer true. For a few hundred dollars you can get a CPU that can executemore instructions per second one of the largest 1980 mainframes. A supermarket chain, where local stores get goods delivered locally, makes localsales and decisions. Best to make decisions at local level, but sometimes corporate level as well.
Many processors still survive if one processor dies.
Add another processor to expand gradually. Both can wreak major havoc. Spread the workload over the available machines in the most cost effective way. Software is more complex. Communication network can lose messages and become overloaded.