Most generations of ABS up until recently only, in fact, have one sensor for both rear wheels, usually located in the differential. This means that, if one rear wheel locks, the brakes will be release on both wheels. I'm not entirely sure why this is, perhaps because of the differential effect?
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black