A bobbin with two independent windings; a magnet spins in front of its iron core...

Each coil will generate a voltage proportional to the number of its turns, both coils being open circuit, no load to them. There is no current on any coil.

Short circuiting the ends of one of the coils, its voltage decreases to zero, current rises.
What happens to the other coil voltage ?

 

The (AC) voltage must drop in the other coil , the first coil is now generating a magnetic field exactly opposing the field from the magnet.

it must be the same as a mains transformer with two secondary coils , ... the primary mains coil is the same as your spinning magnet , generating a field , if one secondary coil draws current this will reduce the voltage in the other secondary coil ...I think