I think what the 教統局 says implies that you can register in more than one DSS. They won't do anything if you register in more than one DSS.
Just like private school, 教統局 won't care if you register in several private schools at the same time.
Of course, 教統局 won't encourage you to do so.
It may be different because a case from my son's elder friend was like that :
Last year, he was accepted by several DSS schools at different period of time. Finally, his parent registered 3 DSS schools & 1 private school until they have made the decision by the end of Aug., 2004.
So, I think we should have time to consider on the condition that we've paid all deposit. We can take it as a reference.