An great explination to clarify the difference between these two systems is given... HERE


Single fire vs. Dual fire Ignition Systems
There is a school of thought that states a single fire ignition system produces a smoother running engine than dual fire ignition. Arguments for single fire ignition center around firing the second plug when the cylinder is not on the compression cycle, the 'wasted spark' theory, the 'wear out the plugs' theory, and the 'not enough spark in the right cylinder' theory. Bike Tech is not aware of any hard evidence to support the "smoother running" statements since this is a highly subjective rating. Since many people want to get a smoother running engine, they will perceive that their motor now runs smoother when a single fire ignition system is installed. While it is possible that a specific engine combination does get smoother, it is likely that more than an ignition module setting was changed. Normally the ignition module, coil, spark plugs and plug wires are changed at one time. Any engine that has a few hundred miles on these parts may smooth out when they are changed.

No difference in horse power has been seen between a single and a dual fire engines in testing of 1994 stock 80 CID motors by Bike Tech using the same ignition and coil to test both modes. Bike Tech tested a DYNA 2000 ignition system with a Crane Single Fire coil in both single and dual fire modes. A single fire coil can be converted to dual fire by simply placing a jumper between the two coil positive leads. Testing under conditions where the only difference was the single fire or dual fire mode produced no difference in horse power. Both horse power curves were identical.


Source: daytona-twintec.com

 

One issue that comes up is trying to add an aftermarket tach to a dual fire ignition is not as easy.