Using keys when unique identifier is a multi node valu

I've looked at the FAQ and can't seem to find an exact answer.  I am trying
to generate all nodes who contain two specific children nodes.  I'm thinking
that using a key would be the best method, but it doesn't seem right to me
to use multiple keys.

For instance, if I have:

<Attendance>
	<FirstName>Bill</FirstName>
	<LastName>Smith</LastName>
	... some other important data
</Attendance>

Assuming FirstName + LastName is a unique identifier, how can I find all
<Attendance> such that Attendance/LastName == LastName being processed and
Attendance/FirstName == FirstName being processed?

Thanks