@iteafreely I don't think there is a limit of one sensor per type, not even for interrupts like door sensors. You just need to make sure you are using different pins and different child ids for the two magnetic sensors. I'm using 8 analog sensors (MQ-... gas sensors) on eight different pins on the same node and it works just fine.
Of course, if you need some interdependence on what the two magnetic sensors report, then you must follow @user2684's suggestion of creating a custom sensor (a C++ class derived from an existing sensor class).
But if you simply want two independent magnetic sensors (for each of the ends of your sliding door), then you can simply create two instances of SensorDoor on two different pins (here pins 3 and 4) and different hardcoded child_id (1 and 2 in this case):
SensorDoor doorRight(node, 4,2);
Unfortunately, both of them will report to the gateway with name "DOOR" (and there's no way to change it without subclassing SensorDoor), but they will have a different child_id 1 and 2 to distinguish the two values sent by the node.
You can then use your smart home controller to handle the two door sensors like one.