There is not a way to do that with multiple anchors. That is the issue with webauth per say, once the client is put onto the other anchor WLC, they have to authenticate again.
That idle timer is pretty high.... usually its best to keep that down to around 2-8 hours. In the new code versions, there is a feature called sleeping clients that you can keep your idle timer down to 300 ms default and use the sleeping client which will keep the client info. Sleeping client only works for webauth and not passthrough or email.
*** Please rate helpful posts ***