Usually Not
The total thrust is usually slightly less. This is due to aerodynamic interference between the two props.
The reasons for doing it are usually:
1. Reduces the diameter of the propellor. It takes a certain amount of blade surface area to absorb the power of an engine. As engines got very powerful, the choice was to add more blades, or more diameter to provide enough area. I've never seen more than 6 blades on an engine (C-130J). Most I've seen in the 1940s: five.
However, if the diameter gets too great, the tips of the blades may achieve supersonic speed at normal rpm. This is a huge loss of efficiency.
Also, in tail-draggers, if the diameter is too great, ground-clearance with the spinning prop becomes an issue when landing/taking-off.
2. P-factor/slip stream. Huge engines have lots of torque and a hideous amount of P-factor. By having contra-rotating propellors (counter-rotating would be like a P-38 or OV-10; contra-rotating is two props on one shaft), the torque and P-factor are neutralized.
Last edited by UAL T38 Phlyer; 05-24-2009 at 10:06 AM.