That question probably should be sent to the TV manufacturer.
I see the same issue on my Sony, it only seems to like one active output at a time.
LG is no help, they say it’s 2 different outputs that will not work together. My TV is out of warrranty, so no help in trying to submit a ticket. I have tried putting SONOS on a separate power strip to shut it off, except it’s a monster pain to get everything in SONOS system working again once its started back up. So that idea is not a plan. What I came up with next is to shut off the SmartConnect in LGTV and then switch over to Digital out so if I stay up later, I won’t disturb anyone, then I can’t use both the HDMIARC and optical out simultaneously, so I have to have the TV louder than most normal hearing people would like. And it shouldn’t be such a hassle to get this system to work! I am at wits’ end. I feel like I have a VERY EXPENSIVE doorstop in this system, it is not working like I have expected. So now what, buy a new TV? I just got that one a couple of years ago! Why can’t everything JUST WORK TOGETHER in peace and harmony?
This box should be able to extract audio from the HDMI-eARC connection between the TV and SONOS ARC.
Edit: I type slow, that box looks interesting.
It really comes down to the TV, if it refuses to activate two outputs not much Sonos can do about that.
You’d have to give up on using the eARC output and go to optical but then you could use an optical splitter/switch to get sound to both devices. I did that with a Beam and my hearing aids by adding the splitter and using the Bluetooth transmitter that came with my aids.
https://www.amazon.com/gp/product/B00HQ2R314/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
For me that works much better than Bluetooth headphones since I haven’t found any adjustable enough to compensate for my hearing loss and my current aids howl under headphones.
That adjustability is also an issue with the Sonos Ace as the simple sliders plus Loudness toggle just aren’t enough to make voices understandable. Still like them as I can listen at night, loud enough to enjoy the sound and use subtitles to replace the voices I can’t usually hear on most movies. Oddly many TV shows work quite well so I have to blame the audio engineer, not Sonos.
I am going to give the optical splitter a try, thank you for those solutions, Buzz and Stanley!