It has been confirmed that you can plug in your PS4 / PS3 / and Xbox 360 into the Xbox One using the HDMI in port. That means that you can snap back and forth between Xbox One and another console at the same time if you wanted to. But there are some more details that you may be interested in hearing before you decide to try this.

Sources:
PS4 feeds into Xbox One
Playing PS4 through Xbox One won’t be a great experience
VGN: Video Games and News
Watch more Red Dragon