I do not think you need two fans. As it is not recommended to speed up the movement of air through the tower. The air must have some time to get in contact with water droplets to allow some of them to evaporate and extract the latent heat of evaporation from them to cool the bulk of the hot condenser water that is supposed to be trickling down to the tower sump. Beside the additional fan will be consuming more power which may contribute in reducing the overall plant efficiency.