Can you define what you mean by "updating each other?" Typically, production and test sites should be placed in two separate installations (file locations) and two separate databases. This would preclude any possibility of file or database data getting intermingled. The only thing you'd need to watch out for at that point would be hard coded URLs that point to the full production site URL. This could cause someone on the test site (for instance) to be redirected to production without noticing.