Fix dual extrusion for Magna Dual

Contributes to issue CURA-6813.

resources/definitions/hellbot_magna_dual.def.json

@@ -28,6 +28,9 @@
         },
         "machine_height": {
             "default_value": 260
+        },
+        "machine_extruder_count": {
+            "default_value": 2
         }
     }
 }