I'm new to the scene so I may not be much help but.....

Advantages might be....
1). If one head fails you can switch over and use the other. (not within the same print of course)
2). Some software utilizes optimization schemes that can improve print time and quality by using the two print heads for different tasks while printing the same object
3). Multi color prints. Can't do that with a single head extruder (at least not near as easily)

Disadvantages might be....
1). Costs more
2). More to fail
3). Second print head can get in the way and mess up a good print