I’ve worked for the same federal agency over 34 years and have never know any policy that required an employee to call two managers when calling in sick. It sounds a little like an intimidation technique. However, even when my manager was in Europe on vacation, we called whomever was acting for her. Maybe the reasoning is if someone is required to call 2 people and they really don’t want to do it, they may just report for duty instead.
If you called my first Federal boss in the evening and tell him you were sick he would say see how you feel in the morning. This was a little intimidation trick he used since you didnt want to get up and call him the next morning.