SQL Server TDE - add master key and certificate
From my knowledge, you cannot have multiple encryption database encryption keys as the Server stores a Server Master key automatically when SQL Server is first installed. It will use this for decryption (among other things) of database keys. 

When you have created a database key (which is implicit) it will store a copy in the Master db, if you issue another create statement that will replace the existing key.

You should be creating a backup of the DMK and storing this somewhere secure, there is nothing to stop you having multiple copies of this key backup.

MSDN should tell you everything you need to know in greater detail.

http://technet.microsoft.com/en-us/library/bb964742.aspx

you will be able to create multiple certificates for external software etc. you can generate these keys yourself or get them from external providers 

http://msdn.microsoft.com/en-us/library/ms187798.aspx