If you are having trouble adding the Saks 5th deal to more than one AmEx Personal Platinum card you may need to make a new account for each card. I have multiple Platinums because I got one by applying for it, and the other Platinum I got when I did a product change from a PRG after one year. I did not have any problem adding The Saks credit to the Platinums that are branded to MS, CS, MB, Ameriprise, or GS.
When I tried to add the second card it kept on switching back to my other card and posting an error. However when I split up the Pt cards to a new account I was able to add the deal. Here is the go by:
Log in to AmEx and click Card Management, then remove card from online management:
Next find the card that used to be your PRG, Gold or Green card, and remove it form your account:
Then log out and create a new account:
You should be able to make a new log in for your second Platinum card as well as add the Saks offer.