let me know if the below doesn’t make sense and i can explain, in essence, if 2 people work together as a joint venture, the joint venture may have to register for VAT not individuals/sole traders. Hence the total sales are considered. and with property, any joint tenancy is by default considered a partnership from VAT point of view, hence the total income of the partnership business is considered.
https://www.gov.uk/hmrc-internal-manuals/vat-registration-manual/vatreg10010