I can't find any reason why the UD2 method shouldn't work.
If you had just replicated the message, as in the snippet shown, so that two identical messages were sent, then you might have received only one update to the client because of filtering, if MERGE mode was used.

Anyway, updates by Event and IndexedEvent are no longer documented since ARI version 1.1 build 22, because, in fact, they offer no advantage over the UD3 method
(though they are still recognized by the Proxy Adapter).