TWF ranger w/ 2 d8 weapons & 18 str + horde breaker + extra attack + sweeping attack + dual wielder=
(d8+4) x 3 attacks for TWF + d8+4 for hordebreaker + 1d8 "attack" via sweeping attack = 33.5. I know TWF is supposed to be suboptimal, but if you are committed to this path this seems like a good combo. Add in action surge for (d8+4) x 2 attacks for level 5 extra attack + d8+4 for hordebreaker + 1d8 "attack" via sweeping attack = 26. End up with 59.5 x 4 times between short rests = 238 in one combat.
Well, the question as always for these combinations is whether or not TWF would be superior to some other option. The core of this combo is Hunter Ranger 3/Battlemaster Fighter 3 to get both Hordebreaker and Sweeping Strike, and then tack on two more levels of either to get Extra Attack.
So, basically, what's been introduced over a standard TWF attack pattern is Hordebreaker and Sweeping Strike, which combo together well in the case of fighting two enemies simultaneously in melee range. Sweeping Strike, which adds one superiority dice of damage to a second enemy if the roll against the first would also hit it, is mostly weapon agnostic. It does favor slightly TWF, because it needs at least one high roll to be worth using, and the extra attack of TWF gives a greater chance of at least one hit.
Hordebreaker, on the other hand, is one single extra attack with your weapon on a second enemy. The math here favors Great Weapon Fighting, getting a 2d6+4 over a 1d8+4. (12.33 avg vs 8.5, factoring in GWF). It also favors Duelist over TWF, 1d8+6 over 1d8+4.
Factoring in Action Surge benefits the non-TWFer over the TWFer, because the extra attack from the off-hand can't be done in the Action Surge.
Of course, factoring in Hunter's Mark always favors TWF for the Ranger, because it's 1d6 per attack. Unfortunately, that doesn't benefit the Hordebreaker/Sweeping Strike combo, because you can only Hunter's Mark one enemy, and Hordebreaker/Sweeping Strike won't hit them.
So, overall, it's certainly a good, useful combo, but not to the point of favoring TWF over GWF or duelist.
