http://www.bernzilla.com/item.php?id=613
In short:
svn log -v --stop-on-copy file:///dev/svn/tags/MY_FIRST_TAG
(get r1)
svn log -v --stop-on-copy file:///dev/svn/tags/MY_SECOND_TAG
(get r2)
svn log -v -r42:79 file:///dev/svn/trunk
(r42 is r1, 79 is r2).