You'd think it to be really easy to find good information on enabling RBS for additional content databases out there. But if you landed on this blog post, then you were proven wrong, like so many others.
Actually it is not that hard.
Once you have followed all the instructions to set up RBS for the first time:
http://technet.microsoft.com/en-us/library/ee663474.aspx (SharePoint Foundation 2010)
http://technet.microsoft.com/en-us/library/ee748631.aspx (SharePoint Server 2010)
You then need to follow these steps for each additional ContentDB:
The important part in the last link is the different msi script. As you use different ADDLOCAL parameters.
Also keep in mind that if you are relying on the FILESTREAM Provider in SQL 2008 R2 you need not install RBS on the SQL Server itself, as it is supported by default. If you plan on using RBS using a third party Provider, then you will need to run the msi on the SQL Server too:
http://technet.microsoft.com/en-us/library/ff629463.aspx (SharePoint Server 2010 without FILESTREAM)
Happy Remote Blobbing!