1. Loud is a relative concept. I have a Greenwing. When she goes off, the whole neighborhood knows about it.
2. You never know with pair bond birds, who they will become attached to. If it's you, and they get jealous of the attention you pay to that other bird, they could attack the other bird. Conversely, if they get attached to the other bird, they could get jealous of the attention you pay to the other bird, and then start attacking you...
3. One one pair bond bird in a pair bond attacks, they often both do. I've seen them turn on people. It can and does happen.
4. Housing in the same cage, (a) depends on the size of the cage, and (b) depends on the bird. I've had bonded conures that COULD NOT be kept in separate cages. I've also seen conures pick on each other. So this is very much hit and miss...
5. A Sun Conure will screech when he does. Suns tend to be "line of sight screamers." If their needs are met, and a routine is established then it shouldn't be too bad. If their needs are not met, or they have "conure spoiled bird syndrome" they will scream incessantly for everything... THAT is more a training thing, and it is as much a train the humans thing as it is a train the bird thing.