Fan dropping voltage?
Most digital pins on Arduinos are qualified to handle up to 25 mA typically and 40 mA at most.  If more current than that is drawn or sunk, the pin voltage will drop or rise, out of specification.  This is due to resistance in the pin drivers, and by Ohm's law, voltage equal to current times resistance will develop.  That “lost” power (voltage times current) is converted to heat within the microcontroller, and can damage it.

The fan motors probably draw rather more than 25 mA; they probably require a few hundred mA.  You can add transistors or motor controllers between the Arduinos and the fans to deal with such current loads.