David and Tina are purchasing a dwelling together. This will be David's first dwelling, but Tina owns another dwelling that she is not selling. For Tina, the new purchase will be an additional dwelling. As, at the end of the day that is the effective date of the transaction, Tina "owns" two dwellings and has not replaced a main residence, the ADS will apply.
The fact that David does not own any other dwelling is not relevant as, for the purposes of the LBTT(A)(S)A 2016, Tina is treated as if she is the owner of the newly purchased dwelling ("deemed ownership").