Google Analytic - Cross Tracking - Two sites, one G/A code

Hi there folks! This is one simple project where I'd need someone to set up one Google Analytic code to track traffic (and everything else) on two different domains. The website has two domains, on one hosting, using the same data, everything. For example: [login to view URL] and [login to view URL] are exact same pages. How hard would it be to set it up and WOULD IT WORK PROPERLY? I've been reading that it's not safe to have this because google might classify it as double/duplicate data and give lower ratings.