You can login using any card number which is linked to this online account with the same password. You can even try that right now - use your PM card number and the existing password, and it should work (even though you had created the password by giving the JACC card number as reference).
Nope, didn't work! My Citi JACC login password does not work if I enter the Citi MP number instead as the username, even though both accounts are linked and visible when I log in with the JACC number.
The online transaction password does not work on the other linked card either.