This wouldn't seem to be an issue at first glance. After all, Multiple Curses of the Elements from multiple Warlocks don't stack, right? Here's the deal though. The Ebon Plague counts as a third disease for the purpose of Death Knight abilities that count diseases to deal damage, such as Scourge Strike and Blood Strike. When only one Death Knight can have Ebon Plague up, the other Unholy Knights are left out in the cold and have a part of their damage stripped away.
Luckily, relief is coming. Ghostcrawler has said that they are aware of the issue and are looking into fixing it. They don't want the debuff to actually stack, of course, but they also don't want to cheat Unholy Death Knights out of damage.
How they'll do it is still up in the air, but hopefully we'll see something like all Unholy Death Knights being able to use any instance of Ebon Plague, or perhaps multiple instances of the debuff on a mob with only one actually provided the magic vulnerability.
There's no ETA on the fix, but for those of you bringing multiple Unholy Death Knights to an instance or raid, it should be some welcome relief. In the meantime, I suppose you could always draw straws to see who gets to respec to Blood.