The issue of council tax adjustments is clearly more complicated than just considerations about individuals. If 50% is appropriate for single occupancy, does that mean that empty properties should have 0%? Few would think that is appropriate, because we don’t like the idea of empty houses. But if you want to boil it down to simplistic statements about cost per person, it would make sense. What about when children grow up and stay living with their parents — do we increase council tax to allow for that? Isn’t that just the poll tax? That was unpopular too. Then, there is generally a fixed number of properties in an area. If they all turned from dual to single occupancy, would we be okay with the local council only having half their previous income?
I think the problem here really lies with the regressive nature of the council tax in the first place. Like I said before, I wouldn’t start from here.