1) Is this referral setup possible between 2 users?
Have anyone done it and can provide their experiences?
Spouse A currently only holds the Amex Plat, while Spouse B currently only holds the Amex Gold.
Spouse A refers Spouse B for the Amex Plat
Spouse B refers Spouse A for the Amex Gold
2) Will the referral bonus still posts even though both users are existing Amex cardholders(as in question 1).
Yes and yes, assuming you haven't already been the primary card holder in the past and are exempt from the bonus. Sometimes you'll see this referred to as 2 player mode. I have done this with my spouse for multiple cards.