Proposed Canon
It is currently unclear if there a variety of titles for different multiple master-class casters (e.g. mastery of 2 vs 3 disciplines, or if "Grand Abbie" indicates explicitly the specific 2 disciples mastered).
"Headmaster" is known to be a title applied to two different casters with apparently different skills in the same class so if "Grand Abbie" follows the same pattern, it would appear to have skill level in either "at least two", "Flower Power/Signamancy" or "Flower Power/Date-a-mancy" within Hippiemancy.
 Real World References
The title is possibly a reference to the title of abbot/abbess.
Mother Abagail is the personification of good in Stephen King's novel The Stand.