All Marketo subscriptions come with the optional Universal ID feature.
To setup users with the same "login" in both production and sandbox subscriptions (or Prod and Prod, or Sandbox and Sandbox) we will want to take these steps:
1. Create a new user in the "1st instance" (first is arbitrary, it is whichever instance you want to start with, and if a user already exists in this instance you can jump to step 3)
2. User is created: lets say for example the user "login" is: email@example.com with "email": firstname.lastname@example.org and password: 12345GoodPassword
3. We will then want to go into the second instance and invite a user with the same "login": email@example.com (you can use a different email or the same)
4. When that user receives the second invite they will want to set their password to be exactly the same as the password in the first instance: 12345GoodPassword
Please note: If after following these steps you want to reset your password, you can do so and would want to do so for both instances. To do so, you would want to input your "Login" after selecting the forgot password option. That will initiate the sending of an email to the email address you specified as your "email".