There's a reason I didn't build this up to level 20 - this is more of a template than an actual build. It pretty much depends on what you value more - the extra spells/abilities from Fey Wanderer, or the damage from Sneak Attack.
That said, I'd personally go for something like Scout 3 - Ranger 5 - Scout 5 - Ranger 8 - Scout 12, but that's going more from a gut feeling than any specific analysis.
That could work too, but you could easily accomplish the same with the Mobile feat, while the extra couple of Expertises aren't so easy to replace.Also i understand that the scout gains a couple of proficiency with skills, but if you plan on dual wielding would the swashbuckler not be a more valid option? The Fey Wanderers Dreadful strikes say that a creature can only take the extra damage once, so if you are duel wielding you do not get the extra damage on the off hand. By going swashbuckler, after your first attack, you can move away without getting an OA and then hit a second creature adding the 1d6 psychic onto that attack as well. Thus assuming you hit on both, youre getting 2d6+dex on both creatures, plus sneak attack on one of those attacks
As for the TWF point, don't forget you have Hunter's Mark as well - getting Dreadful Strikes to go off on a second target in the same turn will be a very situational thing, I'd wager.