How to set multiple objects as children of an object or multiple objects

I am new to Omnigraffle and am wondering if there is a quick way to do this. When working with diagrams is there a way to set a group of objects in the diagram as children of another group of objects. So far I only see how to make one object have several children, but I don’t see how one object can have several parents (i.e. two objects have the same child). Is there a way to do this and if so is there also a way to do this quickly so you can work with a group of objects?

Objects can be multiply linked, but in a directed acyclic graph (outline, tree) there is no meaning that we can attach to a node having more than one parent.