It depends on how you deploy them. If they are located close together (within a few feet), they will add about 6dB to the total sound level. This is a doubling of the SPL, but I have seen that it takes 10dB to give a perceived doubling of sound. If you seperate them and properly place them, you can still achieve about 3dB increase in max output, but the bigger advantage is to smooth out bad nulls in the listening position becouse of different source points and different reflection points.