As for your Insurer, as far as they are concerned, it is more business, more premiums, But? suggest you contact them by e mail, this way you have a record, the insurer will want to know how many bedrooms, Etc, if both properties are [fairly] similar then that should be OK ? a phone call to start then revert to e mails?? If it were me, i would call the two properties 6 and 8 rather than 6/8, the latter could be construed as flat No 6 on floor 8 of a block??
Local Authority?? a phone call then revert to e mails, call Building Control and discuss exactly what they need, depends on local to you interpretation of the "Rules" as they see them?? could be all they want is a full one hour fire door at any breaches in the wall between the two properties,
Local Authority [again] this time Council Tax ?? [spit-spit] as above phone call then e mails, prior to contacting them find out is [say] two mid bands [ one for each property] may be more expensive than one of the top bands?? tends to go on size / volumes.
Assume there will be two doors accessing between the properties, Ground and first floors?? watch out for differential floor levels between each property, been there before !!
Electricity supply?? so long as you do NOT run a cable / lead from one property into another then all should be well, the supply to each house could??? come from different phases of a three phase supply, not a healthy place to mix and match.
Some points for consideration??
Ken.