Eduardo:
This would only be needed in cases where you had two sets of different data
in two doctypes' head elements in the same namespace, and weanted to use the
schema to validate. A good example of this is in an ordering process: a
response to an order requires a reference to the order it is a response to,
which is required. This field does not exist in the order itself, and hence
cannot be required.
You could, of course, make it optional (notice I said "suspect" in my
original e-mail, indicating that this might not be the case...)
Cheers,
Arofan