Internal/External Input/Output system?

[Kickbut101]

Has anybody attempted to make a portable with the capability to output the signal from the console to another screen, play signal on internal screen (like normal portable use), AND accept external input to be displayed on just the screen from the portable (so you have in a sense a "battery powered tv")? If so, how did you do it? I am trying to work it out on paper how to do it with two DPDT switches but I am definitely overthinking or missing something here.
Any guidance?
Thanks in advance

 

[Mario]

I did it accidentally in my Retro 64. All I did was wire the screen's input, the console's output, and the video output together. I had a switch to turn off the N64. So you could have the N64 on, and it would be giving a signal to the screen and outputting it through the video connector. When the N64 is off, the screen is still on but not getting a signal, so you can feed it one through the video connector.

 

[Kickbut101]

interesting... I will investigate further...
Thanks!!