As my back door opens via two doors and can be opened either side at either time, I’m unsure where to fit the entry sensor.
Does this mean I have to buy two entry sensors for the my back door?
If the gap in the middle of both of the doors are quite wide, the sensor may not pick up the notification of either door being opened. If you already have one sensor, try it out and see how far you can split the sensor on each door (probably an inch or two) before it’ll register as “open”.
Attach the sensor to one door and the magnet to the other door. As long as the gap is less than 1.5 inches it should indicate correctly. That way, whichever one opens will trip the sensor.