First: you can not attack with two-handed weapons. Y get the duel bonus
Does mourning exclude the use of two-handed weapons? – Check here if you have questions.
Sacred Weapon: As an action, you can imbue a weapon that you hold with positive energy, using your Channel Divinity. For 1 minute, you add your Charisma modifier to Attack rolls made with that weapon
This bonus is added to the attack roll, not the damage roll. That is, it increases your chance to hit, not your damage.
Also, keep in mind that you need an action to get the benefit, so you can not attack in the same turn you used this feature.
Therefore, the real damage in the hit would be either
- 1d10 + 2 + 2d8 using the versatile property of the longsword
- or 1d8 + 2 + 2 + 2d8 attacking with a single long sword hand.