When adding a second XenDesktop 7 DDC, I ran into the issue similar to the one found here http://support.citrix.com/article/CTX131830.  The article states:

When joining a second DDC to an existing XenDesktop site, the user is repeatedly prompted to enter the credentials for the database though the user trying to add the second DDC has the db_owner permission as per the Knowledge Center article CTX127998 – Database Access and Permission Model for XenDesktop 5

This behavior is noticed when the XenDesktop database is created using SQL script instead of allowing the XenDesktop to create the database after installation.

I had an identical issue when joining my second XenDesktop 7 DDC though I did not create the XenDesktop database using SQL scripts.  The XD database was hosted on the first DDC using SQL 2012 Express.

On DDC1, I noticed that the SQL Browser Service was set to disabled.  If you look at the description of the service, Microsoft says this:  

The SQL Server Browser program runs as a Windows service. SQL Server Browser listens for incoming requests for Microsoft SQL Server resources and provides information about SQL Server instances installed on the computer.

Honestly, being late at night the day before July 4th I was game for anything so I set the SQL Browser Service to “Automatic” and started it.  I went back to DDC2 and attempted to join the existing XD7 site once again and after what seemed like an eternity, though likely 5 minutes or so, DDC2 successfully joined the existing XD7 site.

If you are having problems joining XD7 DDCs to an existing site, make sure to check the SQL Browser Service.  More information on the SQL Browser Service can be found here: http://msdn.microsoft.com/en-us/library/hh510203.aspx