My neighbour was going to this when I lived in a flat. He was going to knock his into mine and create one, large, upstairs apartment.
He then found he would have to pay two lots of community fees as the fees are calculated on the amount of properties. The solicitor also told him he would have to pay two lots of IBI as well. In addition, the water company informed him they would not remove one meter and he would have to pay the standing charges on two water accounts. He didn't bother with the electric company as had given up by then.
Looks like you would need to pay double for everything if you go ahead.
(Incidentally, someone I knew in UK was going to do the same thing. The council said they would only charge one lot of council tax fees but it would almost double due to the extremely high council tax band he would put himself into - it would have gone from Band C at £1227 per year to Band G at £2301 a year.)