Can I Apply Sneak Attack Damage Twice in One Attack?

Question

Is there a rule that states sneak attack damage can't be applied more than once per attack? There are several instances of feats and weapon enchants that allow rogues to gain the sneak attack damage even if they have already gotten it this turn, but nothing specifically talking about if one attack can apply multiple instances of it. Any insight into whether or not the following works the way I think it does would be greatly appreciated.

1. Rogue uses Knockout from stealth, which gains sneak attack damage.
2. Rogue uses an action point, triggering the Slaying Action feat.

Slaying Action: Benefit: If you spend an action point to take an extra action and have already dealt Sneak Attack damage during this round, you can deal the extra damage a second time during this turn.

3. Rogue performs a Coup De Grace against the now helpless target, scoring an automatic critical hit with sneak attack damage from slaying action. This critical hit triggers the Wraithblade enchant.

Wraithblade: Critical: +1d6 damage per plus, and if you have the sneak attack class feature (whether granted by class or multiclass feat) you may apply your bonus sneak attack damage even if you have already done so this round or this encounter. Gaining sneak attack damage in this way does not count toward its use restriction.

Does the Coup De Grace gain the benefit of two sneak attacks? Would both be auto-maximized, or would Wraithblade be a straight roll? Or is it not legal to gain this bonus twice on one attack?

Answer 1

D&D 4e is a game of rules and exceptions, where any specific rule beats the general rule.

The general rule in this case is "Once per round (errataed to once per turn), when you..." (PH1 p.117), right at the beginning of the Sneak Attack (SA from now on) paragraph.

1. Knockout (daily, 9, PH1 p.121) has no special effect about applying SA damage. But you have Combat Advantage and you deal your single instance of SA damage.
2. Slaying Action is an explicit exception. Its text allows you to SA one extra time.
3. Wraithblade is another explicit exception: this use does not count against the use restriction. I'm not sure if this would be maximized damage or not, but it is enough of a different problem to be worth its own question. Go and ask it!

What might be hard to rule is when you score two criticals while using Slaying action and Wraithblade. Since Slaying Action says "a second time" I'd rule the second time to have already be taken from the first Wraithblade activation.

Answer 2

Rogue uses Knockout from stealth, which gains sneak attack damage.

At this point everything is normal. An rogue attack with CA with sneak attack damage.

Rogue uses an action point, triggering the Slaying Action feat.

So another attack (this time an auto-crit) with CA and another use of sneak attack damage. The sneak damage could come from Wrathblade or from Slaying Action but you cannot add sneak damage twice to a single triggering attack.

So, what you could do instead would be to Knockout from stealth with sneak attack damage and then follow that up with a minor action attack like Low Slash (auto-crit and CA) and gain sneak damage from the Wrathblade. Then you could use your action point to ready an action that will be triggered as soon as the next creature starts his/her/its turn. Now that Sneak Attack is once per turn, rather than once per round, your readied action point will technically take place on somebody else's turn and thus be eligible for sneak attack damage. Yes, it is cheesy but it seems to be fully supported by the current rules as written. It makes spending a feat on Slaying Action completely unnecessary. In fact, your DM would need to create a houserule to prevent it.