I want to script a sneak attack on an NPC done by another NPC. SO i need to know two things.
1. What happens with enchanted weapons in a sneak attack? Are the enchantments mutliplied, or only the weapon's damage.
Fairly sure the Enchantment is used as normal, no multiplication.
2. What is the best way to script this? Scripted spell? Use modav health?
Thanks

If your situation is canned (fully scripted situation without any game base AI involved) then you can just use an enchantment with only one charge that will do the Sneak DMG you want.
If its Game AI based just a sneaking NPC attacking another then you can do some scripting on the attacking NPC run a gamemode script that checks GetCombatTarget and if the Target has noticed the Attacking NPC to apply extra DMG or not.
If you explained the exact situation your going for we could probably give better help.