Hi Son,
When it comes to WiNG Express this is designed for simple(r) deployments, hence missing some advanced features like manual cluster setup.
Anyway, WiNG controllers are not recommended to be used in HA (active / active) in order to extend adoption capabilities. When one goes down, you'd exceed maximum number of APs adopted by single controller resulting in adoption loss.
Also, there is an article about cluster members on
GTAC knowledge base
Timo is right - limitation is 2 members for a cluster.
Regards,
Ondrej