Can I fuse two Neutral lines of two different power lines?
This is a terrible idea, not only for a variety of safety reasons, but also because there is no guarantee both power supplies will be on the same phase.  

For instance, I have seen a building with two separate 120/240 main panels, each fed by its own transformer from 480 volt 3 phase.  They had been sloppy about mixing neutrals, for instance a fan circuit ran all over the factory, grabbing the nearest neutral wherever there was a fan, even if that neutral came out of a different panel.  The two panels were originally on different phases of the 480 (for balance) but were later moved to the same phase - clearly they had been having problems.  

A far bigger danger is it could overload the neutral wire (carrying its design load and also the unexpected load), and there would be absolutely nothing to detect this.   Neutral wires do not have circuit breakers.