There have been many free energy videos on YouTube showing a motor driving another motor which in turn generates power to run the drive motor and a load.
I put together such a system to show how a setup like this using two of the same motor will fail.
In this case I used two exact same 3 volt DC motors. One motor is the drive motor and the other is acting like a generator.
First I set up a nice board to hold everything together. Then I strapped the motors to the board.
I connected the motors so that the gears are touching.
Then I hooked up some AA batteries set up in series to give me 3 volts DC. This I connected to the drive motor.
Then I connected a volt meter to the generator motor.
The drive motor was using 3 volts DC while the generator was only putting out 0.65 volts DC.
The reason for this is that in order for a DC motor to generator power equal to its voltage rating, it must be spun up faster than its normal motor speed.
Lets say for example a 3 volt motor runs at 3600 rpm. To generate 3 volts out you have to run it at 5000 rpm for example. Now these are not exact numbers. I just made these up for example purposes. But I hope you get the point.
In the case of connecting two DC motors with a one to one drive ratio you cannot get enough power out of the generator motor to run the drive motor.
You will get less than half the rated voltage out of the generator motor.
The reason for this is because of the speeds stated above but also because there are losses in both motors due to bushing friction and loss in the wires due to heat.
I will later perform the same experiment using a small motor to power a larger motor but I expect the same failure.
With no outside control circuitry and with an unmodified motor you cannot get more power out of the system than you put into it.