My brief thoughts:
1. Most SFF cases do not usually handle a 280. Many (not most) can handle a 240.
2. 2x120 is better compared to 1x240. A 280 is more like 2x140.
3. It is right that using two 120 rads (eg rad->cpu->rad->gpu) can feed each component with cooler liquid than using a single large rad. But the thermal benefit is small to moderate, not huge. The entire system will be in thermal equilibrium but if each component is considered, there is still some advantage individually. Whether the benefit is big enough to justify, it is subjective.
4. I would say, there are other factors that might outweigh this 2x120 vs 1x240 consideration. For instance, ease of building or loop complexity. Using two rads means more fittings, tubings...etc and might increase leakage risk and consume more space (critical in a SFF case).
5. In conclusion, if you go for 'extreme/ultimate' cooling for each component, I would go for 2x120. For all else, just go for a 1x240 (or 1x280), for simplicity sake.