Using a Servo motor as load on a system

Can a Servo motor (or any electric motor) be used as a load on a system ? meaning can it reprisent for example a certain dead weight ?

If we had a motor driving some gears connected to the shaft of the motor, In order to apply a certain load on the system could i put another motor on the other end ? will that burn the motor ? how would the load be applied should the second motor receive current ? or run in reverse ?

Yes, you can do this. You can use a resistor (a power resistor on a heat-sink at higher power levels) as a load on the second motor. Varying the resistance will vary the "negative" torque produced on the shaft by that motor (it is also highly dependent on speed).

You can get higher torque levels by driving the motor "backwards" against the rotation of the shaft.

Either way, the motor rating will be limited to something a little less than whatever it's horsepower rating is when operated as a traditional motor. If the two motors are matched (power-wise) and you are operating at least a little under the rating of the driven motor you should be ok.

If both the driver motor and the driven motor are servos, some have the ability to share a DC bus. This allows the voltage generated by the driven motor to be burned off by the driver motor.
If this is just a temporary system, the braking resistor might be the best way to go, but if this is going to be used often, a shared DC bus might save a lot of money.