I'm fitting some metalclad sockets inside a steel box that's going to house a cctv unit.  It's going to have 2x double sockets.  Someone suggested joining the 2 back boxes together with a 20mm conduit coupler and some threaded bushes, so they are one rigid unit.  

Usually you would put a rubber grommet on the knockout to stop the cable chafing.  If I join 2 boxes together, does it still need something to stop chafing, or is making sure there are no sharp or rough edges on the bush good enough?

Thanks dude