What nobody told me, what I hadn't read anywhere before I bought it was that only some interactions are in stereo. The closest echo will answer regular questions, play podcasts, while the other one just goes inactive.
When you say "Alexa" both normally wake, but as you say "What's the weather?" or whatever, one of them goes back to sleep, and the awake one answers.
If you play music from Amazon's service it will play in stereo. It seems to be some sort of support for different services. If you say "Alexa, play radio 1" (in the UK) it plays Radio 1 from the BBC in mono. If you say "Alexa, ask tunein to play radio 1", then tunein will play Radio 1 in stereo.
This morning I sat at the kitchen table to have breakfast and said "Alexa, good morning". The one closest picked up, and gave me my flash briefing, which is great. Then I asked it to play my morning devotional podcast, and lo and behold the other one (behind me) started playing it.
This is not what I expected when I bought it specifically for this purpose. However, I have to say that when it is in stereo, it really is fantastic, I really like it. I also really like that the volume remains linked in both, so if you use the buttons or use voice control, both change volume simultaneously.
I hope that this mono/stereo annoyance is only teething problems, and that all audio will eventually come from both parts of a stereo pair, but in the meantime, it really is a stereo disappointment.
Edit [1st December 2018]: I got home after a few days away, and one of my stereo pair speakers seemed to have forgotten stuff. I rebooted them both, and one could control the lights, and other skills I had, but the other couldn't. One had access to unlimited music, the other only played samples. When I told it to "discover my devices" it always returned instantly with "no new devices found". I tried quite a few things, but in the end I had to reset it. The only weird thing I noticed was that in the Alexa app when I went into the settings for this device, the volume control was always missing. All working now
Edit [3rd December 2018]: I found this sentence "Note: Paired Echo speakers only support music content" on this page: https://www.amazon.co.uk/gp/help/customer/display.html?nodeId=GPSQQS7N3SBKAEWU
And note: "music content" is different from "Audio player" which is how some of my audio apps work :(
