In AOS CX the remote mirroring is done using a tunnel interface, so the Mirror source and destination must be configured on each Switch. On the source Switch, the source interface (from where the traffic is mirrored) and destination interface (the tunnel interface to where the traffic is sent to). In the destination Switch, the source interface (which would be the tunnel interface (receiving the traffic from the source switch tunnel)) and the destination would be the client where Wireshark enabled client is connected.