Maths:
Yumi double shot min damage (assume hit, 115 tactics, 100 str, 100 anatomy, defender with 70 resist where appropriate)
18
Yumi double shot maximum damage (assume all of the above):
22*2+8*2+7*2= 74
Percentage of hits where either of the above can happen: 6.25% (assuming 50% hit spell) (50% hit spell) calculated 4 times.
Average double hit damage: 46
To put that in perspective, the old favorite complaint, armor ignore, does 35 damage + 8 for velocity (assuming distance) + 7 for fireball = 50 damage and hits for the max 25% of all hits (50% hit spell calculated twice)
After that, if you want to do more damage on average, you need a specialized template to take advantage of certain tactics (i.e. necromancy and 100% poison bows).
All information in the above post is based on test centering and math. No actual players were harmed in the collection of this information.