It looks like I am having the opposite problem. Many people complain that they only can get the same image on their TV and on their laptop.
This is not my case. I can use 2 screens, my laptop screen and the TV screen, using an HDMI cable to connect it. The TV screen is recognized in the control center, when I go to Mint Menu > Control Center > Monitors. I would like to check the box in front of "same image in all monitors" but this option is not available. It is in grey.
Could the problem come from the fact that the 2 screens don't exactly have the same size? e.g in 16:9 mode, the size of the TV is 1280x720 50 Hz and I get a resolution of 1366x768 60 Hz on the laptop.
Thanks in advance for your help.