the order should not be matter.
There are two things.
1. In "Replication Components", you can specify the ACS will send or receive.
So, in Primary, you need check "send" and in the secondary you need check "receive"
2. In partner, If you want this ACS to receive replicated database components from another ACS, the sending ACS must not appear in Replication list of this ACS.
So, in primary, you need move secondary ACS into "replication" box. But in the secondary ACS, it should be leave as blank.
Please refer to configuration document for detail.
http://www.cisco.com/univercd/cc/td/doc/product/access/acs_soft/csacs4nt/acs32/user02/sad.htm#wp909084