Single-Phase Generator
The motor pictured is a claw pole motor. It is a type of permanent-magnet synchronous motor. The inner part has fingers that alternately point in and out of the picture to form alternating north and south poles around the outer part facing the black ring. The black ring is a permanent-magnet rotor that is magnetized for form a series of north and south magnets that face sides of the claws. The black ring is the rotor and the claw structure is the stator.
This appears to be a 16 pole motor. The speed when connected to AC power is given by RPM = (120 x f) / p where p is the number of poles and f is the frequency. Thus the speed is 7.5 x f or 375 RPM for 50 Hz and 450 RPM for 60 Hz. (See below, seems more likely 14 poles.)
The motor should easily work as a generator. Turning it at the one of the speeds mentioned above will result in generating the mains voltage of the frequency for which it is designed. Turning it at a lower speed will generate a proportionally lower voltage and frequency.
The magnetic poles on the rotor should be detectable by carefully moving a small piece of steel like the blade of a screwdriver around the area where if faces the claw structure. When the ring is turned slowly by hand, you should be able to feel that it tends to be pull and stop at certain positions.
The tendency of the rotor to stop or &quot;prefer&quot; certain positions may explain the phenomena that the question describes as &quot;backfeeding,&quot; &quot;running itself&quot; or &quot;getting shock.&quot;
Problems:
Since the output voltage and frequency are proportional to speed, it will be difficult to maintain a constant output voltage and frequency. If you rectify the output voltage, you can connect the resulting DC output to a converter that will accept a variable input voltage and convert it to a constant DC output voltage. A boost converter can increase the input voltage. A buck converter can reduce the input voltage. A buck/boost converter can reduce or increase the input voltage.
As mentioned above, the output voltage and frequency of a motor when used as a generator is similar to the input voltage and frequency when used as a motor. The output current and power available when the motor is used as a generator is also similar to the current and power required for motor use. You should determine the current or power required for the fan. There should be a label stating that. If you can not find the label, try to find the same fan advertised for sale. That information may be there.
The peculiar behavior of test loads may be due to the loads requiring too much current or the generator not turning fast enough to produce the required voltage and frequency.
It will be very difficult to couple together the outputs of two generators. Set that problem aside until you have one generator working as well as possible.
Re comments:
You can assume that the voltage and frequency ratings of the fan correspond to the voltage and frequency rating of the electrical service where the fans have been installed and running. You can determine the number of motor poles by counting the prongs on the rotor and calculating the speed as I did above.   The picture does not show all of the poles. Now that I have looked more carefully, I believe there are 14 poles rather than 16. International regulations require the power of current of a product to be marked on it. That may be embossed in the plastic someplace. There is no way to determine how much power the motor can produce as a generator without knowing the current or power it requires as a motor.
The problem of paralleling the generator outputs should be asked as a separate question after the capability of the power capability of the generators has been determined.