Why are both glucagon and cortisol released in hypoglycemia?
Since the ability to utilize glucose for energy is necessary for life (look at the effect of hypoglycemia on brain function, for example), it makes sense that the regulation of serum glucose concentration would have a number of pathways, that is, redundancy helps to ensure continued proper function should one of the regulatory mechanisms fail.

Glucagon is considered the main physiological response to hypoglycemia; however, epinephrine, growth hormone, and cortisol - in that order - and very probably other factors yet to be named/elucidated also help to increase plasma glucose when it drops.

This kind of redundancy is very common. Cortisol, epinephrine and growth hormone all have other effects on tissue unrelated to glucose as well.

Glucose Counterregulatory Responses to Hypoglycemia