Where to put them? Adding flywheels to the generator shaft was done by Lister on some of the Start-O-Matics. The SOM units also had heavier flywheels. Why both? Two differant problems being tackled, engine pulsations and starting motor loads. Why not just put it all on one shaft for simplicity? Easy, drive stresses. Lets say your after added mass to start your well pump without the lights dimming so you add weight to the crankshaft. when the pump kicks on and the gen shaft trys to come to a stop the extra force that is trying to keep it going is transferred through the drive. Put that added weight on the generator shaft and the drive doesnt know what is going on until the engine governor knows and kicks on some added torque. Lets say flicker is driving you crazy, perfect application for an additional flywheel and it’s handier to put it in the generator head. It will control flicker but the forces again must be transmitted through the drive. If you just had belt chirping under control you just got it back big time.
I guess the simple of all this is put the weight on the shaft where the problem is. Why not over engineering the drive ? This will take all this out of consideration but at what cost. It’s takes torque (fuel) to spin exta heavy belt section or an extra belt. It also takes fuel to spin flywheels. It will take its toll on the belts. Cutting hairs? Who knows? an interesting study for somebody.