No, they don't co-exist well. They will fight over the ndmp port 10000.
I'm not sure about version 5/6, but with Netbackup 4.5 it would not install the client if the BE Remote Agent was installed even if the service was stopped. If you could get both installed, you would need to stop the services of one (either BE remote agent or Netbackup client) while the other is running.