Tornadus-Incarnate is better than its Therian form, but still underwhelming. Base 266 Attack is very good, but it has the same moveset issues that plague its Therian form. A lackluster Flying-type moveset in Air Slash/Hurricane means that Tornadus-I performs worse than Moltres, Honchkrow, Rayquaza, and even Unfezant (the Pidgey of Gen 5), while requiring Rare Candies. Bite/Dark Pulse isn't a bad Dark-type moveset, but Tornadus-I lacks STAB, meaning it performs worse than Houndoom and Absol, let alone the stronger Dark-types like Weavile and Hydreigon.
Compared to Tornadus-Therian, Tornadus-I has much higher Base Attack and lower Base Defense. As a result, Tornadus-Incarnate is easier to duo than its Therian counterpart.
Minimum viable levels assume Best Friends, no Weather, no Dodging.
Italics indicate a Legacy move.
Level 40 Alolan Golem and Weavile fall a little short of duo-viability on their own; the more Rampardos you have, the more leeway you will have to slot in Weavile and Alolan Golem.
Dodging Hurricanes will allow you to cut down Rampardos' death count from 24 total to 10 total, allowing you to avoid relobbying entirely.
Level 40 Weavile, Alolan Golem, Luxray, and Rhyperior are slightly too weak to be duo-viable on their own, but 1 or 2 can be safely slotted in if you have enough Rampardos to get you ahead of the timer.
Level 40 Rhyperior falls a little short of duo-viability on its own, but again, if you have enough Rampardos to get you ahead of the timer, you can afford to put in a couple Rhyperiors and still win.
Level 40 Glaceon is a little short of being duo-viable on its own; again, more Rampardos, more leeway with the timer to put in Glaceon.