These days, more and more young people organize eighteenth birthday party in common. Sometimes on the one event there are two or even three people who celebrating their birthdays.In my opinion, this solution is excellent both for the organizers and for their guests.
The first advantage for the organizers is that the event is cheaper if costs is divided between them. Birthday parties are expensive because they need spacious place such as restaurant or club. So it's good idea to share costs. Secondly, organizing a party alone requires more work and time than when you make combined parties. For instance, one person can take care of decorations and the other person may be responsible for inviting guests. Nevertheless, you might not agree with each other on certain arrangements such as the choice of music for the event.
From the guests' point of view this idea also has many advantages. Firstly,such an organized party there is opportunity to meet new people. I mean, guests can make new friends with people who came to the second organizer. In addition, most young people prefer partying in a club with a large dance floor instead of cramped room in a house as it is usually when there is only one organizer.
In conclusion, to my mind organizing a party by more people than one benefits both organizers and their guests. However, organizers shouldn't be strangers to each other and they should have similar vision of their birthday party.