O'Brian and Bashir got into the habit of recreating battles to fight. The Battle of Britain, with both is flight suits, would mean they were in separate airplanes, since British fighters are one man craft. And the Battle at the Alamo, were you have an entire fortified mission were they could be on different places of the walls to defend it. To the point were they started to map it out on a model to plot out were they should be.
Both those programs would require a lot of holographic people and movement. An air battle over London would take the two all over the place in separate airplanes.
Ha, I forgot all about DS9! Good points. And DS9's holosuites are fairly small as well.