Highlight the channels available for PIP and add option "Open PIP window" to the right click context menu.
Perhaps the highlihgt icon click could open the PIP window either, if possible.
The same for the record, simple example for the main channel list:
[type icon] channel name
[type icon] [PIP] [REC] channel name
[type icon] channel name [PIP] [REC]
These icons should be displayed only for the channels on the same frequency/mux. Perhaps there might be 2 colors for both icons, active/inactive and it could be possible to open/close PIP window and start/stop recording using these icons/buttons.
Another way is to split the channel list and make two channel lists, the other would be for the available PIP channels, the icons might be there too.