Grammar related to "not only ... but also..."

This is not a real parallelism, as the second part of this structure does not offer another element, but simply adds a circumstance to the already existing element. Not only can also be used before the verb. To emphasise that this is an addition and not a parallelism, I would say:

Generally, a system is not only required to be merely stable, but also be stable by some margin of safety. [you omitted this "to" in your second example]

This article shows the way this correlating conjunction is used and says that not all grammarians are so strict about it being used for parallelism only.