How can an Airbus Beluga fly only with two engines and use short runways?
This airplane is based on the Airbus A300-600. The maximum take-off weight of the aircraft is 155 tonne as published in the wikipedia. In addition, the the manufacturer has published the Aircraft Characteristics for the base aircraft. 

If we assume that the aircraft low-aerodynamic aerodynamic characteristics has not been changed for the take-off phase (which the runway length depends highly on the aircraft lift and the engine power, and considering that the wing which contributes to the lift has not been changed and engines remained the same), then we can estimate the runway length for take-off and landing from that "Aircraft Characteristics" documents.

For the aicraft at its max take-off weight, in Sea/level, the required take-off runway length is predicted as 1800 m:


For the landing, the maximum landing weight is 140 tonne. Based on the document, the landing field length is 1554 m.


It should be mentioned that these aircraft are used for transportation of another aircraft sub-assemglies, which are large and light structures, such as wing, fuselage plugs, and tails. For this reason, the take-off and landing weight will be far lower from the above mentioned values, which permits them to operate on shorter runways.