Most objects have more than one force acting on them at any one time. If an object is not moving, it is said to be in equilibrium, meaning that all the forces acting on it cancel each other out.
If both dogs are pulling with equal force, the shift will not move. It will be in equilibrium. Of course, if the forces on it re too strong the fabric itself will tear apart.
Often, the forces acting on an object are not balanced but combine together to have a certain effect, called the resultant. If the direction and size of all the forces acting on an object are known, the resultant can be calculated.
There are many forces operating on this balloon. The force of gravity is pulling it downwards towards the ground. The hot air is creating an upward force called lift. The pushing force of the wind is blowing the balloon along. The friction force of the air on the balloon is slowing it down. If the size and direction of all the forces are known, the direction and speed of the balloon can be worked out.
Picture Credit : Google