Why mark targets?

[MrDiezel]

For Bard, I'll Windbite and Venomous Shot a second target, but mostly for Bloodletter proc against the first.

Unrelated to this post, im a fresh Bard. If you apply Windbite and Venomous Shot to multiple targets, does each application help proc the Bloodletter refresh?? Cuz if so.... thats kinda amazing