You don't need both poles of the magnet to trip this sensor, just the field lines to reverse, which
means having the sensor side-on to the magnet's end.
You can get different varieties of hall-switch, some need both polarities of field, some don't, some
are more sensitive than others, some have loads of hysteresis and some have less (they all have
hysteresis).
Or you can go for a true analog sensor (hall sensors are inherently noisy, so be aware of that)