Two small scrubbers if sized properly would be the same as one large one. The key is surface area, if the combined surface area equals the 5x5 then it's no different. The removal of nutrients would be equal plus it would allow for ease of cleaning, clean one of them a week instead of half of one. With the inclusion of valves between the T and the scrubbers you can shut waterflow off to one to clean it and leave one running, this would also allow for adjusting flow.