In the re-imagined Battlestar Galactica series, the handguns that the military use seem to have two barrels. What are the reasons behind this?
In addition to the advantages that Pearsonartphoto mentions, there are a variety of aesthetic reasons. As a TV show it is really important to have visual elements that identify the world when you see them. Being instantly recognizable, in this case via the double-barreled weapons and various uniform, makes it so casual fans of the show will be more likely to stop and watch as they are flipping through channels. It also helps to make the universe of the show feel more distinct, which encourages suspension of disbelief.
When a TV show is first 'broken' the creators will put together art bibles and other resources that help tie the show's visual aspects together into a cohesive unit. Not only does this help brand the show, but it sets a standards bar that helps the visuals of a show maintain a certain visual appeal.
That appeal could be intentionally slip-shod, which is the case with a lot of the syfy original movies that are striving for a B-movie quality. Or it could be a high bar meant to keep the setting of a show realistic, such as in a lot of HBO programs. It also can define a setting where low tech flotsam and high-tech marvels come together, like in Firefly.
If you watch the season 2 episode where the Galactica has been boarded by a group of Cylons, Apollo is seen to load an armour-piercing round into the lower, larger bore, barrel of his sidearm. I guess that most other weapons are also dual-load with standard rounds and armour-piercing.
Double barrel guns have a number of advantages, they have quicker firing rates, reduced maintenance, and accuracy. Presumably they have some kind of a material that reduces the weight, but the double barrel still gives advantages. For more, see this Wikipedia article.
The upper barrel of the sidearms is for regular ammunition, and the lower barrel is for high-explosive rounds. In Vally of Darkness, the entire final battle is predicated on the use and reloading of these single shot HE rounds to defeat the cylon boarding party. High explosive rounds have been used other times in the series, but this is the only time they are covered by the plot.
Because two is better then one. ;) I should point out that many of rifles of our current armed forces have "two barrels" as well--the barrel for the "normal" ammo and one for the grenade launcher.