Still no response to my FOS complaint. I did send a followup email to the Exec Customer Care Team (or whatever stupid name they have), however it has been ignored.
Also followed up on the phone, but got the Indian call centre who were pretty clueless. Their latest claim is that "yes of course you can have one login for all cards and access everything, but for the travel credit you need to have two logins". Spent about 10 minutes stuffing around that, in that case, one login really can't do everything, but it didn't sink in. Funnily enough, the FOS window for response closes on Monday so I'm surprised Amex have elected not to meet that.
It's a real issue for those with Explorer and Edge cards linked to the one rewards account, as it makes it impossible to access both travel credits.