When is it illegal to tie the neutral wire together?

When we are using analog energy meters in which the reading is taken when the disk rotates which is rotated when the current ingoing from the Live Wire(1Phase AC System) and the same current is outgoing from the Neutral Wire and vice versa. So now there are four places for cable lugs, two for input Live and Neutral Wires and other two for output Live and Neutral Wires. If anyone connects both input Neutral and Output Neutral then Current entering or exiting from the phase will not return back through the meter connected neutrals due to impedance of meter coils, and ultimately follows the short circuited external path. Hence the energy actually be drawn from the source will not be recorded by the meter, so it becomes an illegal act to connect both neutrals.