Twin Disciplines actually activate upon obtaining 19.5 ability power and 9.5 bonus attack damage, in order to compensate for rounding of displayed numbers. However, the 10% bonus magic damage and 10% spell vamp marks are still at 20 ability power and 10 bonus attack damage respectively, meaning that, for instance, if she has 9.7 bonus attack damage, she will have slightly less than 10% spell vamp.