two clients use the same source address persistence table

The traffic is shown as the attachment.

 

First time, client access one VS on BIGIP, the request is sent to one node, after client receive the response date of server, will access the auth server, after auth, the auth server will connect to the second VS on BIGIP, but we need the traffic must be sent to the node that client connects to on the first time, how can I set the iRule to slove it? BTW, we can insert the client ip into the request that auth server sent to BIGIP.