Plex currently does this quite well.
If you have two tuners that are being used. If you wish to watch a channel that neither of those tuners are tuned into, you are prompted as to what you wish to watch instead, and maybe cancel recordings in progress.
See this idea as similar to Sky when you are recording two channels and wish to change channel.
This feature would be very nice for me in a house with access to 4 tuners 