I'm building for my friend TWFigting gestalt Barbarian with high critical range. So far i have build: Brb7 // Martial rogue 6/Swordsage 1, and feats TWF, TW Rend, WFoc (desert wind), Lightning maces, WSpec(Scimitar), Improved Critical, Slashing Fury (probably weapon finesse). Build lacks Shadow blade feat as its high-Dex build, but i'd love to remain in stance Blood in the water. Do you think it will be enough dmg without that feat or i should get it anyway? What PrC and feats would u suggest me? And where can i get from info bout Kaorti Resin?
Moreover my scimitars arent light so far, so i should have either feycraft scimitar or dervish class, both of which makes my build problematic in some way. Can i use monkey grip to make scimitars light?
Monkey Grip will NOT make scimitars into light weapons for you.
Unless your DM introduced Kaoti Resin into the game it does not exist.
Use kukris. Scimiatars don't add enough to be worth it in this context. Also the Aptitude weapon enhancement (NOT the warblade ability) is needed to actually use Lightning Mace here.
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Full keyboard, let me clarify.
A crit-fisher using two weapons needs two things: a huge crit range, and accuracy (to confirm the crits). Kukris give you 18-20 and are inherently light, so the TWF penalties are just -2. Scimitars give you 18-20 as well, but aren't light (so the penalty is -4), and the damage they deal is only 1 point higher on average than the kukris (1d6 vs 1d4). If you score one critical hit, thanks to Blood in the Water, the kukris will match the scimitars, and they'll outdo it with two.
Therefore, just use two kukris. To use scimitars in this manner, you need to be a dervish, which is too expensive, or make them feycraft (in which case you have an expensive 1d4 / 18-20x2 weapon, as opposed to a kukri, which is a cheap 1d4 / 18-20x2 weapon. Huh. Look at that.*) And, as StevenO mentions, Monkey Grip does not work that way, and isn't worth taking at any time.
*Note that Feycraft may still be useful. Feycraft kukris deal piddly default damage, but give you the benefits of Weapon Finesse for free. You said you're a high Dex character, so...
You should also see the Shopping List link in my signature, which goes on at length about gear optimization. Many of those suggestions will benefit a TWF crit fisher, and they're not exactly common choices. I mean, most of us know about the Gloves of the Balanced Hands (buying Improved Two-Weapon Fighting for gold instead of a feat), but not many seem to notice Dessicating Burst (Fatigues on a crit; two fatigues give you Exhausted, which amounts to -3 attack, at least -3 damage, -3 AC, -3 Dexterity save, movement cut in half, and disabling charging or running; that's a killer melee debuff that "just happens" by fighting, with no special action or resource demands on your part) is somewhat less well-known.
With that out of the way, other problems with this build need addressing. Notably, your feats are all screwed up.
1) Slashing Flurry requires Melee Weapon Mastery which you lack. It's also totally not worth it due to the massive drop in accuracy on top of the TWF penalty.
2) You don't have Combat Reflexes, so you don't qualify for Lightning Mace. Additionally, that feat won't work with kukris (or scimitars). The Aptitude enhancement from the back of the ToB will let it work, though, assuming you fix the prerequisite problem.
3) Two Weapon Rend barely rates as "cute", and can probably be replaced with a better feat.
Additionally, I don't know your maneuvers, but you should probably know Sudden Leap at the very least, especially if you don't have Pounce from the barbarian side. If you do have Pounce, there's no reason to not look into Shock Trooper: it makes your charging pounces much more accurate, and adds a ridiculous amount of damage to each hit, even without using a two-handed weapon.
Note: If you're using Power Attack (gestalt barbarian; I didn't see it listed but it's kind of assumed), you'll be better off using a Valenar double scimitar, an exotic double weapon from Eberron; you can't benefit from PA on a light weapon, but a double weapon isn't light. If you're doing that, you should look into the Revenant Blade PrC, which lets you treat both ends of that scimitar as two-handed (dishing out double damage from power attack and +1.5x Strength on each hit) despite it remaining a "light off-hand" for TWF purposes. It's only for Valenar elves though.
And whole complicated feats tree, with conspiracy theory provided, may be deleted 
I was so glad i combined so much strange things 
Anyway great thanks Tempest_Stormwind for such an expanded answer 
Of course im getting pouce, and i'm getting penetrating strike which will let me crit undead x1.5. Soo, how i see it now is to delete whole slashing tree, and delete dervish dip giving light scimitars, combat reflexes is in build already. I will surely grab Revenant Blade; Bladebearer of Valenar will give me "free" double scimitar proficiency, i can treat weapon focus from swordsage for those double scimitar just like for single one, and TWF is natural choice.
Thanks for pointing thoose items i will surely choose them
Now: the player which gonna use that build wished also there PrC Death Delver. I don't see a problem giving it to the build, except i'm gonna spend feat slot for Apprentice (philosopher). Do you see a way to improve this build with use of this PrC, or it will remain only a flavour?
PS. what is better than Two-weapon Rend?