post back to an application

Try this (not tested but you should get the general idea;

when CLIENT_ACCEPTED {
    set fLogout 0
}
when HTTP_REQUEST {
if { [ string tolower [HTTP::uri]] equals "/firstapp/logout" } {
    if {$fLogout} {
        ACCESS::session remove
        ACCESS::respond 302 Location "https://[HTTP::host]" "Set-Cookie" "MRHSession=0; expires=Tuesday, 29-Mar-1970 00:15:00 GMT" "Connection" "Close"
        return
    } else {
         Save request details for use in HTTP_RESPONSE
        set $req [HTTP::request]
         set flag
        set fLogout 1
         Execute second logout first 1.1.1.1 is address of serverA
        node 1.1.1.1 80
        HTTP::uri "/SecondApp/logon/logoff.do"
    }
     }
}
when HTTP_RESPONSE {
    if {$fLogout} {
         Retry original request
        set fLogout 0
        HTTP::retry $req
    }
}