Venus and Jane are buying their next main residence which will require to undergo repair works before it is of a habitable standard. They will not therefore be selling their existing main residence until some months after their next main residence purchase transaction completes.
At the end of the day that is the effective date of that transaction, Venus and Jane, deliberately, own two dwellings and have not replaced a main residence. As a result, the ADS will apply. The date that the ADS becomes due is not related to their move-in date.
However, Venus and Jane will be able to reclaim the ADS paid with interest if they sell their previous main residence at any time within an 18 month period beginning on the day after the effective date for their next main residence purchase transaction.
Details on claiming repayment of the ADS can be found in part LBTT10070 of the guidance.