Semantics of owl:unionOf vs subclass ...

Hi,

I've been trying to figure out how I would write a property and say
its domain can be of type A *or* B. The RDF primer says that
specifying multiple domains is an AND, so that's out.

As far as I can tell, the semantics go something like this:
A subclassof C
B subclassof C
= C is a superset of A u B

C unionOf (A B)
= C is A u B

(then I can say that my property has domain C)

Is this correct? What's the recommended way to specify this?
Thanks,
Ruth