_{Am I interpreting this combo correctly as being able to use the sneak attack twice as part of an Action Point Nova? (formatting is wierding out on me, apologies for the bold)}

Sneak of Shadows [Multiclass Rogue]

Prerequisite: Dex 13
Benefit: You gain training in the Thievery skill.
    Once per encounter, you can use the rogue’s Sneak Attack class feature.

Slaying Action

Heroic Tier
Prerequisite: Rogue, Sneak Attack class feature
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.

Yes a multiclassed Rogue character that already dealt Sneak Attack damage during a round can deal  it again with Slaying Action.

It's probably worth noting, however, that this is a legitimately dreadful way to spend a feat. +2/4/6d6 once every 2 encounters is awful.

It's less bad for use in organized play such as LFR, however, as the bulk of those modules alternate social encounters/skill challenges with combat, such that you wind up with an action point for every combat. In such cases, it's no better or worse than the first 1/enc sneak attack feat.

Battlefield Archers also get 1/enc minimum, but they're pretty feat-starved, and measure damage by the hundreds rather than d6s.

It's a lot worse, actually, because it doesn't come with a skill training and a new class for the purposes of prerequisites.

If a Battlefield Archer is using a sneak attack weapon, he's doing it wrong (the exception being Elves using Treetop Sniper (IIRC that's the name of the feat)).

It's not an awful feat, I suppose, if you literally can't think of ANYTHING else to take, but you should be able to think of something else to take...