"un-uniting" a shape

[Mike Walter]

Is there a method to "un-unite" a shape. Once I've united a couple shapes I
can not seem to un-unite them without using Edit->undo.

Thanks,

 

[David Parker]

That is correct, because you are actually creating a single shape from the
union of others, and then they are deleted.
If you want it to be reversible later, then you have to use the Group
command