Rule 2.9 - Seriously. Don't multiplay.
A good rule of thumb is 'one real person per logged in character per account'. Any more and you're setting yourself up for trouble. If you are multiplaying to gain exp, all of your multiplayed characters will be stripped of experience and possibly any stats you may have gained. You may also have characters deleted, starting with the most powerful. THE ONLY TIME YOU MAY HAVE A MULTI LOGGED IN IS TO SWAP EQUIPMENT.