You definitely need a differential sensor.
Using two absolute sensors is a nightmare.
Let me give an example. How do you weight a baby? Well, if you only have bathroom scales, you pick up the baby and stand on the scales, then hand the baby off and weigh yourself alone. Subtract the two readings.
Not a very good way of doing it because the error in the scales is a substantial proportion of the weight of the baby. You need to have proper baby scales if you want to detect differences of fifty grams or better.
(Actually, this points out another way of doing it - instead of using two sensors, using one sensor and a valve to switch frequently between the two areas to be measured will be more accurate than using two sensors.)