Yes - Arm-by-Location will work even if you leave your 'spare device' always at home.

The automatic arm feature works on 'transitions' through the geofence boundary rather than a steady state inside or outside. So if you leave your iPad at home permanently inside the geofence, it will never pass through a geofence boundary, so will never report either home or away. This means it will not affect the arming or disarming of your home location.