Thanks Steve. A tricky question.
The MSDN library on this is contradicting.
The first article in the links below states
An availability group supports a set of read-write primary databases and one to four sets of corresponding secondary databases.
and the second article states
An availability group supports a set of primary databases and one to four sets of corresponding secondary databases.
http://technet.microsoft.com/en-us/library/hh510230%28v=sql.120%29.aspx
http://technet.microsoft.com/en-us/library/ff877884%28v=sql.120%29.aspx