The double bladed scimitar is significantly better than un-enhanced two-weapon fighting (the combat move, not the fighting style), slightly better than the Two Weapon Fighting fighting style alone, and arguably better than the Dual Wielder feat alone, but completely outclassed by the classic TWF+Dual Wielder combo (unless you spend a feat on your double scimitar as well). On a cost-for-cost basis, it's the best weapon available, but not so good that it makes every other choice pointless.
For the purpose of comparison, I'll assume both attack rolls hit, and I'll refer to the "Two Weapon Fighting" fighting style as "TWF" to distinguish it from the two-weapon fighting combat option.
When you use the two weapon fighting combat move alone, both weapons must be light, and the second weapon does not add a positive ability modifier to the damage; but the double-scimitar doesn't suffer from that drawback because attacking with the second blade is a special ability of the weapon and not the "two weapon fighting" combat move.
Double Bladed Scimitar:
2d4+STR + 1d4+STR = 7.5+(2*STR) damage
Unenhanced two-weapon fighting (twin shortswords, or similar weapon pair):
1d6+STR + 1d6 = 7+STR damage
TWF fighting style alone (twin scimitars):
1d6+STR + 1d6+STR = 7+(2*STR) damage
Dual Wielder feat alone (twin longswords, or similar weapon pair):
1d8+STR + 1d8 = 9+STR damage (and +1 AC)
Based on this comparison, the DBS is better than TWF by only half a point of damage, and Dual Wielder gives slightly superior damage at very low ability bonuses, but once you're at +3 or higher, the double-bladed scimitar is superior. However, taking the AC bonus into consideration, the feat might still be more desirable even at a +5 ability bonus (17.5 damage for the DBS versus 14 and +1 AC for twin longswords).
But of course, the combination of the two completely blows away every other option:
TWF fighting style + Dual Wielder feat (twin longswords):
1d8+STR + 1d8+STR = 9+(2*STR) (and +1 AC)
At a +5 Strength modifier, your damage totals would look like this:
DBS = 17.5 damage
Dual Wielder = 14 damage, +1 AC
TWF = 17 damage
TWF + Dual Wielder = 19 damage, +1 AC
As you gain levels
When you get Extra Attack, you want your primary weapon to be as damaging as possible, so the scimitar (with 5 average damage from dice) pulls very slightly ahead of the dual-wielding options (at 4.5 average damage from dice), but the difference isn't enough to really change anything that much.
But what about Revenant Blade?
You didn't mention it, but the Revenant Blade feat is essentially the Dual Wielder for the Double-Bladed Scimitar, and while it doesn't increase the damage output, it does add the +1 AC you'd get from Dual Wielder. The DBS with Revenant Blade is significantly better than Dual Wielder or TWF alone, but not as good as their combination.
But what about magic weapons?
This calculation changes slightly if you include magic weapons, because a single magical Double Bladed Scimitar counts for both blades, while a single magical weapon only counts for one hand of the two-weapon fighter.
However, that throws so many variables into the scenario that it's hard to compare. In general, a single magical DBS is better than any of the two-weapon scenarios with only one magical weapon, but the two-weapon guy has an extra "slot" to potentially use for interesting magic weapons, so that depends on how rare magic weapons are in your game, and which specific item you're eyeing. A Flametongue Double-Bladed Scimitar would be pretty devastating, but your DM might (entirely fairly) rule that the flames only affect one end of it or something of that nature.
Other Fighting Styles
Since you don't need TWF, other fighting style options are open to you. The Ranger can only really use Defensive with it, but if you're a Fighter, the Great Weapon Fighting style is so good with the DBS that it may well be broken. Your DM might decide to limit the benefit of Great Weapon Fighting to only the primary end of the weapon, or declare that it simply doesn't work with a weapon that, while two-handed, is not a classic 'great weapon'.
The DBS is pretty solidly superior to any other option for the cost, if using your bonus action for extra attacks is your thing, but on the other hand, it's not so superior as to totally obviate every other option. It's only a few points of damage better than other choices, so if you want to use something else, feel free.
Also, remember that the DBA has a significant RP drawback for non-elves. Only an elf can use the weapon without question, and they lack a bonus to Strength; other races will need to win such a weapon by approbation of the elves, which may not be an easy task, or face retribution.