On the City Mini GT2 Double, you can use only 1 car seat at a time, however, you can use 2 carrycots at the same time. But no, two car seats, unfortunately, won't work on that model.
The Donkey 5 does allow for two car seats at once, yes, but you do need the double adapter for that. You need to count on the stroller to be wider than usual with two car seats.
The Thule will, like the BJ, only allow one car seat at a time. The same goes for carrycot - as far as I know, there can be only 1 attached at one time. However, the lie-flat seats with reducer inserts, soft insert cots, or good footmuffs are used for two babies at the same time, if needed.
from these three and your need to have two carrycot / two car seats at the same time, the Bugaboo Donkey seems like the best choice. On the other hand, it does have bucket-style, L-shaped seat units, so that will disappoint you there, I guess.
I would, therefore, recommend TFK double strollers, for ex., the TFK Duo, which, with the convertible carrycots-to-seats as well as the non-reversible seat option, should fit all your requirements, but it will be a heavier model - but very much all-terrain, too. Also, theBumbleride Indie Twin, Mountain Buggy Duet should fit your criteria, and then, the Peg-Pérego Duette Piroet (this one is a bit more urban) and Emmaljunga double strollers (from the newer, the F series is not having bucket seats, older models are all lie-flat).