I may be wrong, but the essence of the text suggests that if you are not married/resident, the tax will be 50% per person, but if married then 100% of the whole/joint.
This is common in CGT (capital gains tax) when 1 partner is resident and the other is non res.
The usual 3% retention for the non res would then be 1.5% and the resident liable to pay their % of the CGT at 9%, being half of the 18%.
I´m beggining to think that a Spanish lawyers translated explanation will make more sense than mine 😯