Just throwing some things out here.
The primary radiator fan is normally turned on and off by the ECU that monitors the actual cooling system temperature using the sensor on the cooling rail above the valve covers. I assume that you will be leaving this system intact.
The Gen I has one cooling fan. The Gen I charging system is rated for a maximum output of 490 watts at 5k RPM; less RPMs than that and the stator output drops off significantly. The implication is that the cooling fan is most likely to turn on when RPMs are at the lowest, resulting in least available stator output. FWIW, Electrosport measured a Gen I charging system and they
claim to have only measured a real 420 watts at 5k RPM.
I have used my amp probe to measure my Gen I's electrical system so I have some real numbers that should be typical of any Gen I. The clamp-on amp probe sends its output to a DMM where the current is read. In the picture below, my DMM is measuring the current leaving the R/R at idle with my heated grips on. It is showing ~33 amps = 442 watts. Since this was at idle the voltage at my battery terminal was around 13.4 volts. With this same setup, at idle with just the motorcycle, normal headlights and running lights I measured 20-22 amps of current = 280 - 308 watts.
The FSM doesn't actually specify the fan motor current so we need to ad-lib a bit. The Gen I fan is backed up by a 15 amp fuse. In most electrical systems the fuse will be roughly twice the expected nominal current. To be conservative, this means the fan motor will draw between 5-7 amps = 70-100 watts. Nominal current draw on my FJR was between 20 - 22 amps = 280 - 308 watts. Add to this the 70 - 100 watts of the radiator fan and we get a wattage bracket of 350 to 408 watts. This is without brake lights. If you are sitting at a traffic light at idle and the fan turns on, you electrical system will see a 350 - 408 watt load right when the stator is outputting the lowest amount of current. If you add another fan (best scenario 70 watts) or two fans (best scenario 140 watts) the charging system will be overwhelmed, the electrical system will be knocked down to battery voltage, and the battery will be supplying make-up current for the stator shortfall. The stator is also entering a zone where it can be permanently damaged.
Connecting any supplemental fans directly to the battery will not relieve the stator when electrical load exceeds the stator output current. I'm not promising stator damage, just mentioning that you are in a serious risk zone. You may get away with this condition until you tire of your spiffy toy -- or not. A volt meter will show you what is going on so you can monitor the health/deterioration of the electrical system. If you are aware that things are starting to go south you can manage the situation and not have to worry about walking home, you will almost always get early notice that the stator is beginning to fail. I'm sure you will be properly wiring any and all fans, so the only real risks are draining the battery and cooking the stator. Time to make risk/reward assessments. Unfortunately, adding a second battery will only cause the stator to work harder. There are not a lot of options to assist the low output of the Gen I's charging system.
Edit: An Electrosport stator is an option. Expect an ES stator to have roughly a 40k mile life, this is actually pretty good for a stator that makes 100 watts over the stock system, nearly a 25% power increase. The other real big buck option is to retrofit a Gen II charging system.