I'm in a new Eberron campaign and after explaining to my DM what kind of character I wanted he suggested a few things. We came out with a Valenar Elf weilding the double bladed scimitar, 4 barb/16 fighter dex build that utilizes the feat for the weapon. My Barb archetype being Bear Totem, and fighter being Brute, giving me 1d4 (up to 1d8 at lvl 20) on both attacks. Due to unarmored defense this build will utilize high AC, beginning with 17 ending at 22.. and the dmg reduction from the 3 rages a day and high hit points allow me to stay in battle whilst essentially doing the dmg of a 2 handed axe per turn. I also want to take the defensive duelist feat after capping dex, allowing me to add my dex mod to my AC once per round as a reaction boosting my AC to24-27,  scaling from level 8 when I acquire the talent, to lvl 20. What this character lacks is much utility outside of combat and traits like sentinal to lock enemies down more and cause the enemies/DM to attack me. What it offers is possibly the best combination of high hp/dmg resistance/avoidance via AC/ reliable Dpr/ and best saving throws via brutish durability not to mention maxed defensive stats con and dex.  At level 20 this character will attack normally 3 times per round and a 4th via your bonus action thanks to the special nature of the weapon, and a 5th due to action surge once per long rest. The damage would essentially look like 2d4+1d8+7×4(5) . I havent even had a chance to test it out but I imagine it'll do the trick.. 

Looks good to me. But I think you misunderstand how Action Surge works. It gives you a complete second action, which includes all the Extra Attacks. So you will attack 4 times, or 7 times if you use Action Surge. This is because Extra Attack says that you attack multiple times as a single action, and Action Surge gives you a whole second action, not just an additional attack.

Thanks for clarifying that for me. Better than I thought, albeit 1/long rest..

You could use it twice as barb 3/fighter 17

I lose the 4th level stat boost from barb in which I would max dex... not sure it would be worth it tbh.. 

Fighter still gives you 6 ASIs and an extra action surge and indomitable is as good as any feat.

What are you expecting your final stats to be? Revanent blade give the double scimitar finesse, so you can leave str unmaxed.

Also, Action Surge restores on Long Rest and Short Rest. I also agree that Barb 3/Fight 17 looks more appealing. You get plenty of ASI as a Fighter so you could skip out on the single one from Barbarian to get the additional Action Surge per rest and Indomitable per Long Rest.

Action Surge

Starting at 2nd level, you can push yourself beyond your normal limits for a moment. On your turn, you can take one additional action.

Once you use this feature, you must finish a short or long rest before you can use it again. Starting at 17th level, you can use it twice before a rest, but only once on the same turn.

After maxing dexterity and constitution, I will just opt for the tough feat. I realize that leaving barbarian at 3 might make an overall stronger character, but since I will likely not be playing until level 20 I'm trying to get a stronger characterAt earlier levels

I can see where 3B 17F is better in the end..  but since most campaigns don't reach 20, It is my humble opinion that 4 levels in Barb first, front loading the max dex at level 4, allowing for the 8th level to be used for defensive duelist, allows the power gains to be fairly even throughout the leveling experience. My DM just killed my last squishy character... and it ain't going down like that again. But it probably will..

Why are you maxing dexterity, and not strength?

Are you not going to use reckless attack and damage bonus from rage?

I wont get dmg bonus from rage, using that just to cut incoming dmg.  Remember the goal for this character is survival 1st with reliable outgoing dmg per round being a nice bonus. Reckless attack is a mixed bag and I wont be missing it a lot. The double bladed weapon with its special ability and feat will make up for much of what I'm missing in rage bonus dmg. At the end of the day, you can't have it all..

How are you doing your AC calculations?

And i'm sure you've been over this if you say you've talked to your DM already, but remember that Brute is UA and not official - a lot of UA content can be either very OP or very weak, as the point of it is to test and give feedback for balancing purposes.

I think my DM has a little tolerance for new content given that it isnt some off the top of the head homebrew type nonsense but is a structured build that is featured, albeit for UA and non official content.

That being said my AC calculations are as follows. Given that I begin with the standard array of 15 dex, 14 con, 13 str (realize this is dump stat but want carrying capacity), 12 Wis, 10 Cha, 8 Int. Given that I am a Valenar high Elf, with +2 Dex and +1Wis and I have the Revenant Blade feat (which he allows me to start with) add a +1 Dex And +1AC. Given that my Dex is now 18 with mod of 4 and my Con is 14 with a mod of 2 Unarmored Defense from Barbarian lvl 1 makes AC 17 at lvl 1. Given a stat increase at lvl 4, maxing dex and bringing my AC to 18.. Fighting style Mariner for +1 to AC at lvl 5.. 19. At 10, 12, and 16 +2 to Con for the Ability Increases, bringing AC to 20,21,22 respectively. Taking the talent Defensive duelist at lvl 8 raises your Ac+5 as a reaction.