Hi,
According to the Pix documentation (command reference, aaa-server) , it says that:
Specifies an AAA server or up to 14 groups of servers with a maximum of 14 servers each. Certain types of AAA services can be directed to different servers. Services can also be set up to fail over to multiple servers.
If the above is the case, then the pix should also look at the next available server if the primary fails.
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_62/cmdref/ab.htm#xtocid5
aaa-server partnerauth protocol radius
aaa-server partnerauth (inside) host 10.1.0.12 xxxxxx timeout 5
aaa-server partnerauth (inside) host 10.1.0.13 xxxxxx timeout 5
crypto map mymap client authentication partnerauth
Regards,
Arul