(Editted) after one hour of searching solutions I just get more and less the practice to just execute a successfully pair stereo, based on another answer found in this site. https://www.googlenestcommunity.com/t5/Speakers-and-Displays/Stereo-pair-connection-issue-with-2-Nes.... At the moment to add the new mini nest just put it in a new separated room. After that, click in the pair option and then the same home google it will tell you where do you want to place the pair device and should work; I spend during the entire process like about 4 hours so this might save time to you.(Editted)
(Original) I bought 2 of this paper stand because they supposed to can be linked as stereo, but first one is always not configured or when I want to cast from spotify there is always the stupid message "I can't connect to the other speaker in this pair. Make sure it's connected to power and wifi." I rebooted, I reconfigured I reboot them, I factory reset and start again and it always is the same result. Google mini nest is a totally fiasco. When are going to fix this? god knows when. and no direct answer either. I probably return this peace of crap directly.(Original)
