I have 1 on my 33L (48x12x12) set on wave 1, speed 3 and the knob turned about 1/2 way. Anything more it creates a wave that real nice for the tank but then the drains start a back siphon from the water rocking too low. LOL Great pump though. One covers the whole tank. I went from 4 powerheads in the tank to just this one.
No dead spots at all. Great flow everythings rocking and loving the flow. 2 wp25s would be complete overkill. Maybe 2 wp10s would have been better considering the size of a single 25 on the side wall, but it works and i love it.
2 in a 48" tank will work just fine. You may not be able to use them at full speed, but it will work fine. I have 1 wp25 and a seio superflow 620 in a 29 gallon. I also have 2x wp25 in my 40 breeder.