Copy+paste is the traditional method. What's unprofessional about it? It's effective because perfect symmetry is important in 5CP. Then all you have to do is go back through and adjust the detailing to suit whichever team owns that half.
It can also be done with instances but there are some limitations with going that route. Generally you'll only use instances for the alpha phase as you're testing layout, and once you finish that you collapse the instances and clean up the detailing.