Healing derived from enchantments (such as Dwayna's Sorrow) is not normally increased.
The fast casting effect of this enchantment stacks with Holy Haste, reducing casting times of healing spells to a quarter of their normal length. Heroes do not use this combo effectively, so if you want them to use this combination, you have to force them to use these two skills in the right order.
Combines well with Ethereal Light for a powerful heal with a small chance of being interrupted.
Consider using Healer's Boon with Heal Party to heal all party members for a significant amount.