Combing two properties for council tax

Hello - We have two properties, one a house and one a flat carved out of some old workshops at the back of a shop around the corner.  The flat and house are joined together with a door, with joint water and Electricty and the entrance on the shop side is never used except for emergencies. They do have different postcodes.

We've asked IOW council if the council tax can be combined, so we only pay one - we can't rent out the flat and it really is another bed/bath room to the main house.

IOW council say to contact the VOA. - but all I can find is information that leads me back to the Council.  How do we joint the properties?  We don't want the title deeds to be combined in case in some time in the future maybe the shop may want to buy the flat from us and we'll need to revert back.

How do we do this?  paying twice council tax for one spare bedroom is very painful! Thank you.

Hello Hellen - postal addresses are managed by the local authority and it reads as if you need their street name and numbering dept to help alter, change delete the postal addresses involved for you.
That may mean losing the 'side door' for example or something else
Certainly not a HMLR thing and I'd imagine the Valuation Office are only going to be looking at 2 properties whilst 2 actually exist