ORIGINAL: taltamir
While only one of my ship can world on a world annihilator project, i have seen multiple AI ships work on one... and even constructors from multiple empires all working on it together (specifically, i saw one being worked on by 2 constructors from empire A, 1 from empire B, and 1 from empire C... i shot all of them and then took it myself).
I could tell they were all working on it because they were all emitting sparks, and they covered different "portions" of the world annihilator (my constructors always go directly to the center, they were on the outskirts of it)
I've found this isn't 100% true. If one of your construction ships is already working on the world annihilator, no others will be able to be assigned to it, but if you order several of them to repair it while there aren't any ships already doing so, multiple ships will move in and repair simultaneously.