As has been said before, it's all about coil saturation. The longer the magnetic field has to build, the stronger the spark when the field collapses. The field is only building when the points are closed (the "dwell"). If it were as simple as just setting the gap smaller (so the points barely open), you'd be done. Unfortunately, the smaller the point gap, the more likely they are to arc - you need a minimum gap to avoid the possibility of this happening. Dual point distributors utilize a "make" and a "break" set of points. If either one is closed, the coil is getting juice (effectively increasing the dwell). Only when BOTH points are open does the coil spark. Each set of points are set to a wide enough gap to discourage arcing, but are staggered as to when they open and close. When the "break" set opens, the "make" set is still open (and the coil fires). Shortly thereafter, the "make" set closes (starting the dwell). The "make" set stays closed until the "break" set closes. Once that happens, the "make" set can open (but the coil continues to saturate, since the "break" set is still closed). Then, the "break" set opens, the coil sparks, and we are back to the start of the cycle.

The dwell runs from the instant the "make" set closes through when the "break" set opens.

Decent electronic ignition renders this all moot.