You urls are not in the same repository, so you can't do it with the svn diff
command.
svn: 'http://svn.boost.org/svn/boost/sandbox/boost/extension' isn't in the same repository as 'http://cloudobserver.googlecode.com/svn'
Another way you could do it, is export each repos using svn export
, and then use the diff command to compare the 2 directories you exported.
// Export repositories
svn export http://svn.boost.org/svn/boost/sandbox/boost/extension/ repos1
svn export http://cloudobserver.googlecode.com/svn/branches/v0.4/Boost.Extension.Tutorial/libs/boost/extension/ repos2
// Compare exported directories
diff repos1 repos2 > file.diff