I just did this, Double Cash with Premier. I had to change all of the telephone numbers and email addresses to match, but one account had my middle initial and the other did not. The combination appears to have worked as expected.
Thanks. That's a good tip. I've had this problem for a long time and finally fixed it by updating all the phone numbers.
I tried to call Thank You Rewards customer service but got hung up by the system 3 times in a row without anyone picking up.