We're venturing into the same waters at the moment. We will be adding a second black capped conure in about 2 weeks in the hopes of giving our little billie some birdie companionship. I think it depends on the particular bird though, have you ever had the opportunity to have loqui around another bird? I've noticed with Billie that he really likes his human time but he also seems to want to interact with our other birds but they are all either too big and want nothing to do with him, which can be dangerous, or they are all too small(the budgies and p'lets) and they are afraid of him and fly away when he tries to get close. I'm hoping that if he has another conure around that he may have better luck with making friends. Now, on the other hand, our only other "single" bird is our alexandrine, ziggy and I don't think I would ever try to get her a "friend". She just seems content to be a loner. Occasionally she will want to be held so I will pick her up but it usually only lasts for about 5 minutes or so and she wants back on her cage and she has made no attempts at trying to be friendly with our other birds and despises if any of them attempt to get on her cage.
As far as mess/noise, more birds equal more mess. If you plan to keep them in the same cage then you will probably have to clean the cage twice as often, I know with our amazons, they require much more cleaning than any of our single birds but they are also much larger. Right now, we have 13 birds and 8 cages(5 cages with pairs and 3 single bird cages) and it really doesn't take me that long to clean. I've come up with a pretty good system though, I try and have paper cut for all of the cages so I don't have to do it each time I clean, we use a big roll of white paper that I cut to size, it's a little more time consuming than newspaper but I think it looks really nice and it's nice and thick so two pieces per pan keeps anything from soaking through. I have a spray bottle with vinegar/water in it and I spray one thing down while I'm cleaning another, and rotate pans and grates in the shower on full blast.
They will most likely be noisier as well, when you have one bird he may talk/whistle/scream for a few minutes but without encouragement they usually stop and move onto something else. With multiple birds, as soon as one starts going, more usually start so it's a bit louder. If your house is generally calm and quiet it shouldn't be enough to really bother you unless you are overly sensitive to noise, which I doubt you are since you own a bird already. Our little birds are actually noisier than the bigger guys. Their volume is obviously not comparable but they tend to chatter a lot. I was afraid when we brought the zons home that it was going to be a lot louder but the only time they really get going is when we encourage it, if we talk or whistle or sing to them, they usually start getting pretty loud but all it takes is for us to calm down and get quieter and they follow. I even play music a lot during the day, not too loud but loud enough, and they usually just sit quietly and listen along.
I think when adding another bird you have to go into it expecting them to never get along, be prepared to have to have separate cages and separate bonding time. That way, if they never do get along at least they will both be happy. My plan is to eventually have the two conures cages next to each other and just see how they interact. If they get along and like each other than that is great and hopefully at some point they can "move in" together but I know that it may not go that way and I am prepared for that. Sorry for such the long post, I tried to keep it short but there is just so much involved and there really isn't a right way to do a lot of these things, it's whatever works best for you and your flock, the main thing to keep in mind is safety. No matter what you do, make sure it's in the best interest of your birds, no you, not your ears and not your wallet. It sucks sometimes but it usually pays off in the long run
