Working with two systems requires manual daily or weekly integration monitoring.  I would recommend calling QuickBooks Sales and discuss which QuickBooks online product is best for you and possibly you can switch from Enterprise completely.  

In the meantime, if you are truly interested in having two accounts and maintaining two accounts.  Recommendation would be set up a system to export data from online to import data into QuickBooks enterprise.  

This may not be relevant but many of my clients depending on what type of business you have, will have a separate software that integrates with QuickBooks online.  Example, a service industry that dispatches technicians has a separate software where technicians and office personnel log in, customer communication, invoicing, estimates and payment collection.  This then is integrated with QuickBooks online and will either automatically sync or manually sync the relevant data for accounting.