Well, ideally they should be in a completely different building. The closer in proximity and less time you use to quarantine, the chances are better that you will eventually get cross contamination. It can be something as "minor" as mites to something more deadly like IBD and many things in between. Then again, I'm sure there are people out there that have never practiced quarantine procedures and have never had a problem. In the end, it's up to you to decide what you're comfortable with.

My advice would be to keep them as far away from each other as you can for at least 3 months.