How do I create one parent polygon for two existing boundaries?

In a case such as Katmai National Park and Katmai National Preserve, since they are administered as one complete unit, I would like to add a parent polygon that encompasses these two: “Katmai National Park and Preserve”. What is the best way to go about doing this?

You can create a relation and add the two as members.

No objection, just interested: What would be the benefit of another relation including the same objects? The National Park relation already includes a wilderness area relation which is fine, as the areas are different from each other. But just adding the 2 neighbouring areas together in a new relation would not add anything new, even the foreseen name is just an addition of the 2 existing names.