In "Basics: Part 1" when the voyager fights multiple Kazon factions, they created multiple holographic fighter ships to distribute the shots from the Kazon ships so that Voyager becomes under less fire. When they first initialise these holographic ships, the doctor appears in space stating something along the lines of "I told you we should have run a systems check beforehand", indicating that these holographic ships run under the same computer program as the doctor. This proves that it is infact possible for there to be multiple copies of a hologram running simultaneously. I assume that this never happens in the show for the same reason that transporters cannot create copies, or replicators cannot be used for cloning, yet they can be used to create organs.
There are tons of contradictions like this in the Star Trek series. Stuff like that is inevitable with science fiction, although Star Trek does a very good job at being scientifically accurate as often as possible (where the narrative allows anyways). In one episode of Star Trek (TNG I believe) they used a transporter to get rid of a virus in one of the crew's body using a backup from the transporter logs. If this was truly possible, don't you think they would have done this more often? Such as healing injured crew members, or even bringing them back from the dead?
Anyways I came to this question cause I was wondering if there was any defined reason that I had just missed. This rant was merely a side-effect