From what I see the latest version of OmniPlan is totally broken for the leveling resources capability when multiple resources are assigned. This is very easy to reproduce. A simple example is attached.
In this example, “Dual Task 3” should start as soon as Resource 2 becomes available. Instead, it waits until Resource 1 becomes available as well, despite setting the activity to “Resources work independently”. This makes the app useless since you can’t level resources.