@Lauris & Gene As a new host, I too hoped to have been able to set up my wife as a Co-host, although in my case (a few weeks ago, when the system then was obviously working) an email was sent to her email address, BUT........
.....what you probably assumed (as I, and possibly many other hosts' might have!) would be a simple process of 'nominating' one's spouse or partner to be your"Co-host", actually isn't what Airbnb are 'talking about!
Put into simple English (as I sometimes get carried away in explaining things!) a "Co-host" to me means someone who works alongside me, who could take over if I was ill or absent, or to whom everything would default if I suddenly snuffed it (i.e. died!)
In Airbnb-talk however, a "Co-host" is a friend-who-lives-down-the-road, or round the corner, or someone who might be willing or prepared to (in your absence) attend to your bookings, contact and correspond with your Guests, welcome your Guests when they arrive, attend to any of your Guests when they might have a query, and anything else that the 'title' " actually involves!
Having said that, unless the Host decides to make ex-gratia payments to her/his/their "Co-host", or to set up a regular payment to him/her, the 'position' of "Co-host" isn't designed to receive any financial reward for doing any of those 'tasks', and furthermore, s/he isn't able to place reviews or responses to those of Guests in her/his own right!
Needless to say, although I did comment to Airbnb about what I feel is a misclassification of the term "Co-host" (no reply/feedback received), I cancelled my request for my wife to be a "Co-host", as although she's also my best friend, she neither lives down-the-road, or around the corner, and she's fully my "Co-host" in everything bar Airbnb's own meaning. Furthermore - and I can't see how Airbnb could disctate or act otherwise - should she wish to access my account, and do everything and/or more than a "Co-host" is allowed to do - she has my full blessing.