Im not nervous about online purchases, in fact if it were not for RockAuto.com, my Z would probably be scrap.
What I intended with my previous post was DONT WASTE YOUR TIME WITH A MC FROM NAPA ONLINE, (very poor personal experience).
As for your situation, looking at the 79_280zx_wiring, the low-level signal will be 'active' if either reservours are low. Both of the sensors are basically tied to each other up stream. You should only need to use one sensor,