In almost all games, multiplaying is okay - in the sense that you are allowed to have more than 1 character.
However, a common rule is that you may not have 2 characters online at the same time. Your characters also usually cannot interact.
The reason for this is because in these games with 2 characters you can kill another player unfairly, or even just gain levels unfairly. Most importantly, you can use one high level character to get all of the good equipment and give it to a lower level player.