No need to max twin arrow really. It’s more of a choice.
Do you want more damage on it early on? If so then level it. (+150 multipliable damage per hit *5 is quite a bit at lower levels)
Or do you want it for extra utility? If so just leave it at one.
Also 100% + skill damage? It’s fine and all to use Dream’s method of simplifying it, but make sure you know how everything else works with it.
And, both hits deal the same damage on the skill sim video, so unless they changed the 2nd hit to deal less damage, they both deal full damage.