Indeed, the proof is trivial.  Let a be the root pattern of schema A, and b be the root pattern of schema B.  Then <choice><ref name="a"/><ref name="b"/></choice> matches any document that matches either a or b or both.  And since there are no constraints on the subpatterns of a choice pattern other than those inherited from the context of the choice pattern (which here is null), all such patterns are valid.