2 Demoman is a problem
2 Medics is a problem
2 Snipers is allowed in NA (even though they dont have good snipers xd) its not a problem.
2 Engineers is not a problem, you can just spam a sentry down, and then the other. They dont have time to rebuild the sentry so...
2 Pyros is not a problem, even if you put 2 pyros on 1 door to deny uber, you dont have a flank (my flank can just go in and spam the pyro from behind, or go point or w/e) and they might both die and you still lose. Your stickies will damage both pyros, and if you just bring in a solly behind the combo you can spam them both to dead.
2 Heavies is maybe a problem, uber 2 scouts in to kill the first heavy, but then theres still 1 heavy left somewhere, but you should have gotten control of one side of last and you have 2 more faster classes so you can just spam the other heavy to dead.
2 heavies + 2 engineers is a whole another problem though, or
2 engineers + 2 snipers, its almost impossible to spam the sentry without getting sniped.
etc..
I think it could work, but we never tried it (with current meta and teams) so who knows...