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 masteries (Thinkamancy/Lookamancy vs Thinkamancy/Findmancy?) so if "Grand Abbie" follows the same pattern, it would appear to mean mastery of 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.