Two plugs are used on engines for many different reasons. For aircraft engines it is for reliability, if one ignition system quits then the airplane keeps flying. Many pilots think it increases power, because the engine RPM drops when you check the mags by turning off one mag. But actually using two plugs changes the timing. Because there are now two flame fronts the flame spreads faster and it has the effect of advancing the timing. You can regain most of the power by simply advancing the timing on one cylinder, there may or may not be a very small power loss after adjusting the timing. On racing engines it is usually to increase timing on very high RPM engines, to a point that cannot be done with one spark plug. On nitro engines it is to allow a richer mixture of nitro, and to keep it running if that mixture is off.

But for the old glow engines it was to allow two heat range of glow plugs, one for idleing, one for power. Not sure it worked very well. It often improved idling, but still lost power at full speed.