Add a second critical path

I have two major milestones on my project which would benefit from each having a critical path.

I can add a dependency and delay between them to force the earlier milestone tasks to show in the critical path, but ideally I would like to be able to show it as its own critical path.

Is this possible?

No, sorry. Critical path in GanttProject consists of all tasks with zero float. There are no hidden features, and no magical tricks.