OK, I tried again tonight.
From the Alexa app in my browser (desktop), I was able to invite my wife who typed in her Amazon account (members of the household you invite need to be on the same country as the host)
I noticed some funky behavior the 1st time (ie entering the Configuration mode) which I canceled.
The second time it went through though and I got a successful message within the Alexa app.
I am able to switch accounts vocally just by saying "Alexa, switch accounts" but there is no way to link my wife's spotify account to my Echos in either the desktop website or the smartphone app.
After googling some more, it seems like you cannot have household members use their Spotify on Echo devices which are linked to the original owner's Spotify account (not sure I'm clear lol).
https://www.reddit.com/r/amazonecho/comments/95rqzz/can_i_use_the_same_spotify_family_account_on/Apparently that's 1 Spotify account per Amazon account ie per device (each member of the family would have his/her device with his/her Amazon account on it and his/her Spotify account linked to it)
But I want to keep my Echo devices all under my Amazon account for management reasons and multiroom capability obviously.
Shame
[Edit] OK, I think this whole thing might work with
Amazon Music because I have a dropdown list with my name and my wife's name under :
https://alexa.amazon.com/spa/index.html#music/PRIME_MUSIC/stationsX's music library
Y's music libraryBut neither of us are Prime member to test this