you just need to enable it on both servers and then use a client (eg Spark) which is logged in “im” and try to talk to “foobar@im2” or use the conference service conference.im2 (if you did set up DNS (SRV) records) of the other server.
Wildfire will then try to create a s2s connection to “im2”. If something goes wrong you’'ll see errors in the log files.
So if conferencing fails you should add a DNS record for “conference.im.domain.local” and “conference.im2.domain.local”. Just like jabber.org and conference.jabber.org.