So, I tried out paired pistols with laser sights at the range today. Fortunately the range owner is a friend so he humored me.
Firing a 9mm Glock 17 and a Taurus PT840 .40, each with lasers and properly sighted in, at a 10 yard range at human-shaped paper targets spaced 20 feet apart:
2 targets, 2 pistols, no lasers: Damn that's difficult. I ended up really focusing on one and then the other much of the time, really. Missed most shots.
2 targets, 2 pistols, with lasers: Easier but still a pain. I could get some body shots on both targets but any more precision (like specific body areas) was about impossible.
1 target, 2 pistols, no lasers: Moderately difficult as sighting down the barrels of two pistols required more things to keep track of than usual. Missed a number of shots.
1 target, 2 pistols, with lasers: Again, easier. I could consistently group within a one foot circle. Which is much worse that my usual single-pistol groupings, but hey.
Each test was performed with 6-8 shots per pistol.
Really, my biggest observation is that when firing with iron sights, you have to keep track of at least three objects, the rear sight, the front sight, and the target. To hit you need to line up all three. With the laser sight you only have to keep track of two, the laser dot and the target. Aiming using laser sight is almost instinctive, rather than the conscious thought that goes into lining up iron sights.
In all cases, shooting with the laser sight was easier than shooting without the laser.
That was a fun twenty bucks worth of ammo to spend.

-k