I guess splitting the systems that way means that one of the two wheels compared by the antilock system is (if necessary) released by the hydroplaning protection. That way it spins up and provides a reference for the antilock system. Effectively, the antilock system extends the hydroplaning protection to the other wheel.
Presumably that makes hydroplaning protection on all four wheels unnecessary. So maybe limiting the authority of the hydroplaning system to two wheels is a failsafe measure (so a malfunction can't cause loss of all braking). Or maybe there would be undesirable interactions between the hydroplaning protection and antilock systems if both acted on all four wheels, though I can't put my finger on exactly how that would happen.
Actually, a much simpler answer is probably that the hydroplaning protection isn't intended to protect any particular wheels, but rather to prevent the situation where the antilock system doesn't activate because all four wheels are turning at the same reduced speed (or not at all). If the problem you set out to solve is that all four wheels may turn at the same speed, the last thing you'd do is design a system that acted on all four wheels in the same way.
Last edited by Chu Chu; 28th Jan 2017 at 02:08.
Reason: Added last paragraph.