And what race would that be? You haven't provided the race, nor any actual stats. Why did you choose each class in the first place? Also, what feats do you have? What weapons were you thinking of using with tempest? Why do you want to go TWF in the first place? Keep in mind that weapons are rather expensive so having to rely on two of them is going to eat into your wealth.
Tempest is a very bad choice for a mounted attacker. Remember that it's your mount that moves, not you, so you get no benefit whatsoever from Spring Attack. Taking all 5 levels when you won't even benefit from the capstone of getting two-weapon spring attack is a very questionable move. The bonus to AC from using two weapons is pretty crappy and definitely not worth the feats and levels. Getting to apply feats from your main hand to your off hand is alright, but TWF takes far too many feats to get going and be worthwhile without some sort of bonus damage like Sneak Attack factoring in to help mitigate the lower weapon damage compared to going with a two-handed style. Considering you're using the Feat Rogue variant, you don't have any SA damage to help out in that regard. Though that's by no means a bad thing since the feat rogue is an awesome ACF because getting fighter feats is worth more than SA damage anyway.
Dervish requires more levels than 1 to really be worthwhile. If you want to use a scimitar then perhaps using the Great Scimitar from Sandstorm would work, and then go into Exotic Weapon Master from CW to get 2x strength to damage with it instead of the normal 1.5x strength to damage for wielding a weapon in two hands. In only one level of EWM and with far fewer feats invested you'd be getting the same damage through fewer attacks than if you'd spent five levels in tempest.
One of my suggestions is to entirely drop dervish and tempest. On a mounted build they are worse than worthless to you because you could be taking other classes that have much better synergy with that style of combat. That'll free up a lot of feats and class levels so you can actually do mounted combat.
On a mount build you can get some nice stuff for your mount by being a multiclass paladin/ranger and using the
Devoted Tracker feat, and then going into Wild Plains Outrider.
If Dragon mag stuff has any potential to be allowed, there is an alternate combat style for rangers from Dragon 326 that grants Ride-By Attack at 2nd, Spirited Charge at 6th, and Trample at 11th instead of the usual archery or TWF feats. If Dragon MAg stuff isn't allowed, it might be worth asking your DM if the style substitution can be used.