to separate the HH and th O is easy, just takes a few volts. The problem lies with producing enough to be useful. Its a Very slow process, so unless the unit has been running for a few hours, and contains a compressor to store the gasses produced with enough pressure to produce a hot flame for the time needed to cut or weld, then its not practical.
Also the energy required to make usable quantities is pretty high, (you need a mains supply, and regulated voltage supply) so the costs start to mount.