You could try the following wiki syntax, it works for me on 1.17
# one
#:<pre>
#::some stuff
#::some more stuff</pre>
# two
It is not perfect, because you end up with a more indent but it does allow one to use the wiki syntax for correctly formatted pre blocks over multiple lines.
As previously mentioned, the other proper way would be to use HTML mark up.
<ol>
<li>one</li>
<li>two</li>
<pre>some stuff
some more stuff</pre>
<li>three</li>
</ol>