Given the system of OOB we have been using lately, I don't see a problem with 2 or more DN's flying together. If you have a limited number of them on the board and choose to concentrate them in one area, you will be leaving other areas without DN support.
Also, there are some situations where DN's flying together may be required to deal with an extremely tough opponent, such as a squadron of BCV's. Could the Alliance have ever hoped to take on WarSears' B10 in GW3 without grouping dreads together?
Considering how easy it is to draft someone you don't intend to, I don't think such a rule would work well even if we wanted it to. What happens if a DN accidentally drafts an allied DN, along with one or more enemies? Are you going to penalize him?
Finally, multiple DN's tend to get countered by multiple DN's (or other heavy metal), which lead to some truly fun and epic PvP battles.