I see how I can "Add Guests" to an individual event (so, I can use that to send a note of a specific family birthday to my wife's email as well as to my own associated GMail; or indeed to one of my own other email addresses...), but I don't see how to do it for that useful Daily Agenda email......
The only place you can access that Daily Agenda option is from SETTINGS > NOTIFICATIONS, where you can simply turn it on or off.
What I'll also try is sharing the whole calendar (SETTINGS > SHARE) with my other non-Gmail address - maybe the Daily Agenda will get sent as a result of that sharing enabling. Either that or the GMail forwarding filter mentioned above will work......
AGB