SSML is not specific to VoiceAttack. VoiceAttack merely passes the information to the text-to-speech engine you select.
Which SSML tags work, depend on the specific text-to-speech engine, and would have to be documented by its author.
Dynamic response sections on the other hand, are a VoiceAttack-specific feature, and are parsed before the SSML is submitted to the text-to-speech engine.
So if you want to have multiple potential phrases, or sections of phrases, that works just like it would without SSML.
E.G.
<speak version="1.0" xml:lang="en-US">
[This is a <prosody rate="10%">test</prosody>;This is also a test]
</speak>
I.E. the header only needs to be present once, wrapping the text that is to be spoken. Other tags can wrap said text, sections of said text, or individual words. Whether they need to be repeated depends on what you want them to affect (E.G. in the above example, the word "test" would only be spoken slowly for the first potential phrase).