The question is not which one works better or it would work or not. Efficient means the over all performance that is with less maintenance, less power losses, less head aches and etc. Of course having 2 skimmers may have better skimming but they may not be consuming less or equal power as the single bigger unit. Assuming all units are on their optimum settings, the time dwell of bubbles depends on the pump capacity. With 2 lesser capacity pumps, each can only skim as much.