THE PROBLEM:
If you want to connect multiple accounts. Once u connect the 1st one, the subsequent accounts connections will just be defaulting to that 1st account, hence would not allow you to connect new accounts.
THE SOLUTION:
1. Connect the 1st account.
2. Open a new browser tab, access twitter.com, logout of Account A.
3. Login to account B that you want to add to Postly.
4. Go back to Postly and add new account, the new account B will show up.
5. Repeat the process.
6. This can also happen by logging in to all your Twitter accounts, then switch to each one connecting it to Postly.