One-light between two-screens: Makes better use of the light, but leaves the other sides of the screens unused (in the dark), thus wasting half your flow. The big advantage is cleaning: You can clean one screen, and leave the other in operation, which give you more consistent filtering.

Two-lights on one-screen: Makes better use of the screen (both sides are lit), but can waste light if not reflected properly. Advantages are (1) redundancy of the lights: If one goes out, you'll still have filtering until you can buy a replacement, and (2) higher performance for its size, since each side of the screen gets hit by light from both sides, especially right after cleaning when the algae is thin.

Best of both worlds: Multiple lights between two screens. Uses the most flow and power, but is always filtering, and will never go totally "dark" unexpectedly.