The elevator in front picks the nose up by lifting the nose. The elevator in back lifts the nose by pushing the tail down. The elevator in front moves down to increase lift in the nose. The elevator in back moves up to push the tail down.

For pitch stability, the center of gravity must be ahead of the aerodynamic center of the whole aircraft. The bigger the forewing is compared to the aft wing and the greater the distance between them, the more forward the location of the aerodynamic center of the whole model. Calculating the location of the aerodynamic center of the whole aircraft is not a simple task. The simplest way to find the answer is to build a small, exact scale model glider of the model you are designing and adjust the CG of the small scale glider until it glides well and then measure the CG location. It will have the same relative location in the bigger model.