Your interpretation is correct
You can only apply Sneak Attack once with Double Slice - and you'll have to be flanking your opponent either way (or have some other means of them being flat-footed).
When to Double Slice?
There are many hundreds of permutations of relative AC, attack bonuses, etc., and all of them will vary depending on what percentage of your damage Sneak Attack is. In some cases, Double Slice will be useful; here's some rules of thumb:
- When your second attack with MAP is unlikely to hit anyway, Double Slice will generally be better; if you're only going to hit once, the penalty of not being able to apply Sneak Attack twice won't be noticeable. This is most likely against higher-level boss-types, shield/heavy armor types, and other targets with high AC (or something like invisibility).
- When your second attack with MAP is likely to hit, Double Slice will be worse.
- If you don't have a flat-footed target, Double Slice is just better. An obvious case, but an important one to acknowledge.
If you have some specific situations you want to look at, feel free to comment and I'll delve more into specific math. Things that would be helpful to know:
- Are you a Thief Rogue that is maxing Dexterity?
- Are you getting your Fundamental Runes on level? Earlier? Later? Or are you using Automatic Bonus Progression?
- For your Property Runes, are you grabbing damage dealing runes? What kind?
- Are you grabbing any Critical Hit rider effects (from Runes or otherwise)?
- Are you grabbing any special Debilitations (importantly, like Precise Debilitations)?