Dual-wielding Scythes is a *really* bad idea.
Your attack numbers will absolutely suck, and you won't be able to hit anything.
Here is the break-down. Note that you will ALSO have a -2 penalty for *both* Scythes with the required Monkey Grip feat - so "add" that on as well.
Dual-wield - NWN2Wiki, the Neverwinter Nights 2 wiki - Races, classes, skills, and more
Monkey Grip - NWN2Wiki, the Neverwinter Nights 2 wiki - Races, classes, skills, and more
Toss in your level 2 penalty and call yourself the "whiff master" or the more dignified "Lord of Air", because that's all you'll hit (..and I don't mean air elementals).
Choose either:
A Scythe two-handed, or
A Scythe with Monkey Grip (and -2 penalty), and a large shield, or
A One handed weapon of your choice with the feat two-weapon fighting and either a light weapon in your off-hand (only a -2 penalty both weapons), OR with a *non*-two handed weapon (-4 penalty both weapons). Obviously a light weapon in your off-hand is preferable.
Additional problems with 2-weapon fighting:
1. It only gives you *1* extra attack (in addition to the other penalties).
2. It requires a 15 in Dexterity, and chances are you want Strength as your primary attribute. Worse, the additional "enhanced" Two-Weapon fighting feats requires even higher levels of Dexterity.
Considering that penalties are a problem for actually hitting anything - it is advisable to use light weapons IN BOTH HANDS. More specifically Kukri(s).
Further, consider the *RANGER* to "get around" the problems with Dexterity (..they have a two-weapon combat specialty feat that requires you to use light armor, but gives you the feats for "free" and does NOT require a high dexterity.) Also, a Yuan-Ti has this as their favored class so it doesn't count for XP penalties.
Optimally you should consider *6* levels as a Ranger to pick-up the 2nd two-weapon fighting feat: Improved Two-Weapon Fighting. (..a second off-hand attack at a base of -5 is bad, but at higher levels it can still hit on occasion.)
Ranger - NWN2Wiki, the Neverwinter Nights 2 wiki - Races, classes, skills, and more
What additional classes you add to this really depends on how you want to play the game.
1 *Option*:
THE consummate dual-wield class is the Invisible Blade.
Invisible Blade - NWN2Wiki, the Neverwinter Nights 2 wiki - Races, classes, skills, and more
It has several things going for it:
1. High base attack bonus
2. Intelligence modifier AC bonus (..particularly useful to augment your light armor).
3. Feint-based Bleeding Wound can add a LOT of damage to an opponent that isn't sneak attack immune.
It requires some extra things - Bluff 8, Combat Expertise and Feint (feats), and a requirement to *always* increase your Bluff skill to the maximum at level-up (for your feints) - essentially requiring the feat Able Learner in addition.
IF you want to add-in the RDD build then it would work something like this:
Yuan-ti:
1st level Bard w/ feat Able Learner (always increase Bluff at each level)
2nd level Ranger
3rd level Ranger with combat style 2-weapon fighting, additional feat: Weapon Focus Kurkri (required for IB)
4th level Ranger
5th level Ranger
6th level Ranger w/ feat Combat Expertise (required for Feint which is required for IB)
7th level Ranger (you will need a Lore skill of 8 to choose the RDD class next)
8th level RDD
9th level RDD w/ feat Feint (required for IB)
10th level Invisible Blade
11th level Invisible Blade
12th level Invisible Blade (feat of your choice, Power Attack or Improved Critical Kurkri)
13th level Invisible Blade
14th level Invisible Blade
15th level RDD (feat that you didn't choose for 12th level)
16th level RDD
17th level RDD
18th level RDD w/ feat Cleave
It can carry-over into MOTB with RDD and then back to either Ranger or Bard and still work well (..better as a Ranger to Ranger level 11).
Intelligence should be 14, Charisma should be 14, Constitution should be 14, Strength should be 17+, Dexterity should be 12+ and Wisdom - whatever works-out (bearing in mind that you probably won't be able to cast your Ranger single 1st level spell per day without at least an 11). (Note that it isn't even worth trying for Ranger spell casting.)
That is just one option among many.