Dual zone A/V receivers work VERY well in my experience. While it is limited, I have set this up in one (and only one) location, but it works really well.
The A/V receiver passes video + audio to the second zone TV, this means that it tells the source to switch to 'stereo' instead of surround sound. I think you will still have to make analog audio connections from the source equipment to the receiver, and I'm not sure how it handles the actual audio output if you only have a digital device, like a Roku or Chromecast product. But, if it is a cable box with analog audio, you won't have any issues at all.
Stereo audio will be fed to the speakers, as you have already been doing, in zone 2, and video will be sent to zone 2 TV over HDMI. Zone 1 TV can watch something completely different. If both zone 1 and zone 2 are watching the same thing, there will be a LOT of flickering for about 10 seconds as copyright protection is authenticated, but then both displays will show the same thing.
The hard part is usually getting the HDMI cable outside to zone 2. You can typically use HDMI extenders.
This is all predicated on a 1080p system, not a 4K system. 4K looks like it will work as well, but you will need the proper cables.
