For some reason I don't have problems logging in with different accounts as long as I open 5 different browser windows or 5 different bots. You can try that out too.
Just do this but with 10 tabs (I'm using Chrome). You'll have to be extremely fast. All you have to do is select the world that you want on all of the tabs and then click "Play" within a short time frame.
I'm sure if you have them all in the lobby with the world selected, and only the play now buttons showing on your screen, you can spam click them all. But remember if one logs out, to get it to log back into that world you have to log them all out.
- Go to your admin folder on your computer.
- Find the file Random.dat
- Right click the file, go to Propertys.
- Click on "Read-Only". Click on Apply and you're done.
I've done 10 accounts in 1 world on 1 computer. 10 is the maximum per world using this method.
Goodluck
EDIT: Just looked at RwmRS2 his links, its pretty much the same.