To add multiple accounts (from the same platform) add the first account following the instructions and click "Connect". Once you've added your first account you can follow the same steps again to add another account.
If you add more than one account for the same platform it is important to give the accounts nicknames. This will help you differentiate between them within Affluent. If you don't choose a nickname, Affluent will assign default names, e.g. Awin 1, Awin 2, Awin 3, and so on.