Ok, here you go.
1. Load up 2 browsers, go to the same detail and world on both (low detail runs better)
2. Make the windows small and put them side by side so you can see the login on both.
3. Enter the username and password in both of the windows to your accounts.
4. Click one login, then click login on the other really fast (within 1 second) and it'll work.
Trust me, this works.