Can a unit have abilities from two SUB-JOBS

It´s posible. The first two skills an unit has are available regardless of the sub job you currently have. Those are like the main job primary skills. Now if you look closely, their icons have a special border that the other skills don´t have, to help distinguish them. There are other skills that also have these borders, so they are also main skills and can be used regardless of the job you have equipped. Ayaka has curaga as one of these skills, and i´m guessing she was using time mage to use comet.