One fundamental presumption is that the normal power source is going to be fairly reliable, and that the emergency power source will not often be called upon to function. Another fundamental presumption is that you will not have a simultaneous failure of the normal power source and the emergency power source. Indeed, there are testing and other requirements that are intended to minimize the probability of a failure of the emergency source at the one time you need it to be functional (i.e., when the normal source is lost). Therefore, there is no requirement that a second emergency source be available, just in case the normal source is lost and the first emergency source picks that moment to fail.
Although having a second backup generator is not an NEC requirement, nothing in the NEC would prohibit it either. If the owner wants to spend the money to obtain a second level of emergency power, they are welcome to do so.
For my part, I have seen it done in only one type of facility: a nuclear power generating plant. They have a requirement that they have to be able to safely shut down after an incident, without relying upon the normal utility power source, and with the presumed failure of one single component. Since the emergency generator is included in the list of "designed single failure" components, they always have at least two emergency generators.
