getting both the fate chalenges has rewarded exactly 15% in the past if i remember rigth (7,5% for each) i think similiar to the dungeon chalenge (also 15% togehter but i'm not sure about the 7,5% each)
if it gives 84,5k at 57 i assume both rewards will sum up to 169k exp of 3kk which would be 7-8%

seems like they halved the reward...