Ben and Antonia live together in rented accommodation. Ben also owns a buy-to-let property.
They now decide to buy a new main residence whilst keeping Ben's buy-to-let property. At the end of the day that is the effective date of the transaction, Ben and Antonia will own two dwellings.
Although they have purchased a new main residence, they will not have disposed of the ownership of their previous main residence (as they lived in rented accommodation) therefore the ADS will apply.