They have already answered this. Yes, you can plug a 360 into the HDMI In. What they didn't answer was the controller use. I don't see how you can use the XB1 controller. You'll be required to use the 360 controller. BTW, they even said anything with HDMI Out would work, even a PS3/4. LOL
As for Live subscription, they answered that one too. You only need the 1 subscription for both.
Tommy McClain
Thanks so its like having two machines in one really if you use your Xbox one HDMI in in this way .
Because of the quick switching between both ......a good selling point as well in my opinion .....well at least for the first year or so .