The gemara in Berachos 8b teaches us:
כל האוכל ושותה בתשיעי מעלה עליו הכתוב כאילו מתענה תשיעי ועשירי
One who eats and drinks on the ninth day of Tishrei in preparation for the fast the next day, it's as if he fasted on both the ninth and the tenth of Tishrei.
Ie that one who eats alot on Erev Yom Kippur it's as if they fasted 2 days and get credit/ rewarded for doing as such.
1) Since we say that Yom Kippur is "like" Purim- ie יום "כ"פורים
2) and Purim has a din of "ונהפוך הוא"(switching around/opposite) namely, switch around Yom Kippur's day of eating ➡️ Fast, to Purim's fast ➡️ then day of eating
Can we assume that whomever fasts on Ta'anis Esther it's as if they celebrated/ get credit for two Purims?