At the scales used, twin-bladed propellers operating at considerably higher rpm than those on the full sized aircraft can generate more than enough thrust for the aircraft. Full sized aircraft used 3-, 4-, or even 5- bladed propellers because they needed that many blades to effectively turn available torque from the engine into thrust at the rpm's that the engines and props could operate. RPMs were limited by both the engine and the maximum speed of the propeller tips, so more blades were required. Longer propeller blades could be used, but could have strength/durability issues, and would require measures to ensure ground clearance (e.g. the inverted gull wings on the F4U). Using two bladed propellers is simpler and cheaper for the model makers, and you can't see them when they're operating in any case.
Mike
