If you intend to install two drains instead of one, the results of the calculator cannot simply be halved, because, for example, the cross-sectional area of two 1" pipes is only 1.57" while the area of one 2" pipe is 3.14." In other words, two 1" pipes cannot handle the same amount of water as one 2" pipe, and, in fact, will handle far less. The correct formula to convert the results of the drain calculator for use with two pipes would be: the diameter returned by the calculator is represented by the letter d. sqrt((((d / 2)^2 * pi) / 2) / pi) * 2, so a 2" pipe would equate to sqrt((((2 / 2) ^ 2 * pi) / 2) / pi) * 2 = 1.42". Rounding up, two 1.5" pipes could handle the same flow as one 2" pipe.
Ok, unless you are a math freak, you probably tuned out pretty quickly on the last paragraph. That's okay, there is a good rule of thumb that alleviates the need for the mathematics. All that needs to be done is to reduce one standard PVC size down from the results of the calculator and you should be safe. In other words, one 1.5" pipe could be replaced with two 1.25" pipes. Two 1" pipes could replace one 1.25" pipe. Even better would be to use two drains of the size recommended, which would allow one to be completely clogged, but the system could still handle the full flow without missing a beat.
Got this off the reefkeeping magazine website.