Dervish needs 4 feats to enter. One, Mobility, can be gained from the armor enhancement I mentioned to save you a feat.
TWF is ultimately 3 feats long.
Two Weapon Defense is a horrible set of feats. The bonus is tiny for each feat spent, and as Grogg himself said, an animated shield will eventually obsolete them. Even if you can't get or don't want a floaty shield, the Improved Buckler Defense feat is plainly superior, too! At least once you start magically enhancing the buckler.
Rogue can help. For one thing, there is the
Martial Rogue, aka "feat rogue." Which you can actually multiclass w/ Fighter 1 or 2 to maximize feat gain. Even regular rogue is nice for 4 levels to get uncanny dodge and such.
Rogue 4 / Fighter 2 is a perfectly decent way to go towards entering Dervish. Your sneak attack will be small if not doing feat rogue, but it's still some extra damage, and just
having sneak attack can open doors. Like the Craven feat, which just straight up adds your hit die amount (ie, 6 at rogue 4 / fighter 2) to sneak attack damage. Or Staggering Strike feat, which forces a save vs. staggered every time you do melee sneak attack damage.
Do note that Dervish, upon giving Spring Attack, means you are well set up to grab Whirlwind Attack. Now, normally WWA sucks... But a Dervish can use her dance to move and full attack along the move. WWA is a full attack action. It's just so awesome! Of course, you're doing TWF, and WWA is much better suited to a single, 2H weapon...
You also should absolutely be getting Elusive Target. It ups your survival by a lot, and if you obtain Improved Trip, you can generate a LOT of extra attacks w/ Cause Overreach -- neither the trip it grants nor the bonus attack improved trip grants are attacks of opportunity, so you have no real limit to how many you can gain in a turn.
Feats you'll want:
All Dervish pre-reqs
TWF, ITWF, and GTWF
Elusive Target
Feats you might want depending on exact build/plans/hopes and dreams:
Craven
Staggering Strike
Whirlwind Attack
Deadly Defense
Improved Trip