In the past when I have had to do this the 2nd registered guest name would appear on spg.com "my stays" confirmation/summary screen. It would show something like Guest 1 "My Name" and Guest 2 "2nd guest name"
I noticed an upcoming reservation did not show the 2nd guest name so I called and it was in fact already added.
Has there been a change where they aren't listing the 2nd name on spg.com?
Haven't checked the app yet but that doesn't usually show as much of the reservation details as I would like anyways.