FMS drop existing playing user and play new one (live stream)

Hi,

I'm in need of the following:

1-User A connects to FMS to the live application, and plubish a stream named "streamA".

2-User B connects to FMS to the live application, and publish a stream named "streamB".

3-Clients play a live stream named "main", which switch to the last live stream that has been published. If there is not any live stream, the "main" stream simulate a live stream from recorded files.

Any ideas? Thanks in advance.

Kind Regards,

Robert