They would obviously have to be pretty week if it was something like there was two killers. I think its a good way to maybe lower the amount of survivors, adding an option for duo queues. Perhaps even something like Cho'Gall from HOTS, where both people have different abilities but control one character perhaps with two heads and one also controls the walking ... Which is probably easier to balance than having two weak killers walking around. An alternative may be a new game mode huge map, with 2 killers and 8 survivors who have to do more gens.