You should be fine for any clam.
Whether they have similar PAR ratings per inch isn't as important as their collective intensity. In other words, when you have a coral under a 24" lamp, it's getting light from all 24", as is the coral under the 48" lamp.
The length of the tube doesn't really matter. A 24" t5 will give Tue same par as a 48"t5 the only number that matters is the depth.
How long have you had these clams?
Can you clarify this? How is the total energy of a 24" tube being delivered to a single coral? The PAR is spread out along the length of the tube. So if you could concentrate all the light from the tube to a single spot, then that spot would get the tubes total energy, but without concentration that PAR is spread out along the tubes length. A simple reflector concentrates the light around the circumference of the tube, but the light is still distributed down its length. Right? One reason LEDs with lenses can focus the energy better. But even with them the PAR is distributed across the panels area of illumination.
In the clam example with a LED panel, it may be just a hand full of LEDs in the panel illuminating the clam. The rest are lighting the surrounding area. So PAR is also a function of delivered light density.
I think that helped. The first comment confused me. It's a good calculus problem as the coral is illuminated from multiple angles. The hot spot from a corals point of view still would be directly perpendicular to the tube (shortest path) overhead. So a specific organism such as a clam, would benefit more from multiple parallel tubes than one long tube. The tanks length dictates the tubes length. But 4-24" tubes arranged to cover 48" would be the same as 2-48" tubes.
+1 It may also safe to say that 4 24 in-tube in a 24" tank will have the same PAR as 4 48 in-tube in a 48" tank, provided both are the same type and are mounted at the same depth.So a specific organism such as a clam, would benefit more from multiple parallel tubes than one long tube. The tanks length dictates the tubes length. But 4-24" tubes arranged to cover 48" would be the same as 2-48" tubes.
Now what about an anemone for my lights? Other than BTAs, what else are considered "easy" anemones? My tank is coming on 6 months old, thinking about getting one in a couple months!