Flow rate is dependant on several factors. How much water the u-tube can handle, how much water the bulkhead can handle, restrictions, water drop distance, yadda yadda yadda. The biggest factor on these overflows is that they have a large bulkhead. If you put an additional u-tube on something like a 3/4" bulkhead, it can only handle so much water. Think of it like this, you have two buckets. One has a 1/2' hole and one has a 6" inch hole. Which one can you poor water into faster without it overflowing the top? Make sense?