I went to aTV Control Panel and switched on the feature where I send audio to BOTH my TV and my HomePod. Of course this first encounter was pretty disastrous with echo effect, so I went through the rigamarole of calibration (for both 60Hz and 24Hz) and now the audio part is very sweet! I get a whole room full of audio, synced together and with the TV. All is good, and all works well with Apple's TV.app playing various content.
BUT not so good with Channels DVR, playing content recorded and stored on my mac mini. That's always worked fine, and basic playback continues to work fine, everything as expected. But anything that involves random access is something of a disaster. Backward or forward jumping (30s fwd, 7s bwd) are VERY slow, while commercial skip is basically broken: the video seems to jump over the content to the next segment (good) and then just stop (not so good!) It's unclear what has happened. Has playback paused? Is Channels.app very confused? After every commercial skip, I get this, which looks like a hang, and playback won't resume till I hit the play pause button a few times and kinda "wake up" Channels.app.
This is all very sad, ruining the magic that I felt when I got the whole room filled with balanced audio rather than flat audio concentrated where the TV is 
I haven't seen anyone complain about this so perhaps this is such a specialized situation that no-one has put together this set of features yet? If others want to try there are a bunch of non-obvious things I had to do to get the audio working properly, and that might be why no-one has hit this before.
So: I have a 3rd gen (non-4K) aTV. Running tvOS 13.3.1
I have a HomePod running whatever the latest HomePod OS is (I updated when iOS 13.3.1 came out).
aTV and macMini are connected by ethernet. I don't see any reason why network issues (wired or wireless) or network delay and (mac mini SSD disk delay) are relevant to the problem.
Step a - go to Control Panel in tvOS (LONG press of the home button on the aTV remote). Move down to the AirPlay button and you should see your HomePod as an option with a o round thing next to it, while your TV is ticked. The o round thing is AirPlay UI design language for "this speaker can play SIMULTANEOUSLY with other speakers", so select the HomePod, move the selection up so that you can be sure BOTH your TV and HomePod are selected, then go to the home page and start playing some TV!
The first thing you will now notice is that the HomePod and TV audio are out of sync. You need to fix that!
Go to the Settings App, find Audio and Video, go right to the bottom, and select Audio Calibration and do what it says. Basically you will put your phone mic close to the TV, the aTV will make various crazy sounds, your phone will hear them, and the system of phone+aTV will measure the delay it took for your TV to generate them. Once this is done the TV will say Audio Calibrated and (in principle) everything is good. BUT BUT BUT ...
Apple UI let the team down at that point, right after some clever engineer got all this technology working because there is a huge problem here that is not obvious.
Do you play ALL your content at a forced TV frequency (of probably 60Hz)? Or do you have your aTV set to "Match Content" which means the TV is told to toggle between 60Hz and 24Hz depending on the src content frame-rate. (And perhaps, though I've never seen this case, also to toggle screen resolution?)
IF you have Match Content on, all you have done so far is to calibrate audio for the default settings of your TV (probably 60Hz 1080p). You now need to go to the aTV video settings, choose every other different resolution that you use (I chose 1080p 24Hz, which is buried deep in an "Other video resolutions" menu), make that the default aTV resolution, and go through audio calibration again!
Once you have done this for every Content Setting of interest (for me this was 1080p 60Hz for TV, and 1080p 24Hz for movies) NOW you have magical simultaneous audio from multiple speakers.
And it is at THIS point that you discover that Channels.app falls apart when doing random access, especially skipping over ads.
I hope I have given enough detailed steps here that other people (especially the Channels DVR testers) can reproduce the issue. If necessary I can try to film the delays with my iPhone so you can see just how bad/unusable it is.
