What size are the driver's for each subwoofer? Will they be placed into different positions within the room? Ideally, if multiple subs are used, they should be identical subwoofers. They can be paced accordingyly throughout the room to balance response and increase SPL's. If different sized subwoofers will be used, they should be crossed over appropriately and probably placed in the same location. This could lead to potentially better response and higher SPL's but is not really worth the time, hassle, or money spent on appropriate crossovers.
For intsance, say one has a 12" and an 8" sub and really wishes to use both. The 8" sub woofer should be crossoed over lowpass from 100hz-80hz and high-passed at around 40hz-50hz. The 12" sub would then be low passed at around 40hz. In effect it would be like having a 2-way subwoofer. In this instance the 8" and the 12" should be placed in the same ideal loaction specific to the room. This should not be attempted without the use of steep crossover slopes of 24db/oct. or higher. The only way I could see doing this would be if one were using an 18" or larger driver for all but the lowest frequencies (30hz and down) and a 12" for frequencies from 30hz-80hz. This would require seperate amplifiers for each driver as well.
It should be noted that a 12" woofer would be fine running lowpass from 100hz-80hz and down and the above scenario, really, is not necessary.
