Well, if you need to buy a bigger cage for two birds, the cage you have for each one is too small because these little birds need lots of room to fly inside the cage (they are flighty which is another characteristic that differentiates aviary and companion species).
I am not explaining myself correctly because it seems to me that you did not understand me so let me try again. Aviary species do not need to be tamed to trust you, they do that all on their own if you give them enough time (like any other animal). I told you I have aviary species as well as companion species that had not imprinted to humans which makes them pretty much like an aviary species because the ONLY difference between the two is the bonding and a companion species that has not imprinted to humans will regards humans the same way that an aviary species would - and these birds are perfectly at ease with me, they do not shy away from me or my hands, they perch on me, they preen me, etc.
As to how do I know that a budgie or a cockatiel will not be happy alone... well, it's actually pretty easy: science tell us! Species are classified into many different groups with one of them being their social structure (which is actually much more complex than just social or territorial -see the links I give you below, there are all kinds of nuances) and we know that science (I am talking about etiologists, field biologists, ornithologists' observations, studies, field reports, etc) classifies parrots as 'highly social' which means that they evolved to live in groups. When an animal lives in groups in the wild, it needs to be kept in groups in captivity for it to be happy. This is not my personal opinion, it's a fact and the reason why you see social animals kept in numbers in the zoos and why it's actually illegal to keep a dolphin or a whale by itself in captivity (which should apply to parrots, too - like in Sweden where it's illegal to keep one single parrot alone). Animals that are deprived of what they evolved to need develop stereotypic (abnormal or aberrant) behaviors (look it up, you'll see - it happens a lot with elephants). Now, even within a highly social structure, we can find differences. Lovebirds are highly social but they are pair-oriented while budgies and tiels, which are also highly social, are flock-oriented so even though it's not ideal for a pair of lovebirds to be by themselves, it's not too-too bad while for tiels and budgies, even when you keep them in pairs, they will feel the lack of the flock more strongly than a pair of lovebirds would. A perfect little flock for captivity is three bonded pairs (I had flocks of over 30 budgies, 30 lovies and 30 tiels in the rescue). I only have a pair of lovebirds now because I got them fairly recently and, in truth, I was not going to take in any more little ones but it was a special situation and I felt bad for the previous owners so I took them along with the two older, larger birds I had originally agreed to take. I switched them to a good diet, got them to trust me and then released them in the birdroom so they can live cage-free (and you should see how happy they are in there!) - and now I will have to look for other lovies that need rehoming so as to 'make' a small flock for them. As to why I keep them? Because captive-bred birds cannot be released to the wild or they die. They cannot survive on their own because they were never taught survival skills by their captive-bred parents (parrots are all highly altricial and need to learn everything from their parents) and this is not their natural habitat. Somebody needs to care for them - and that's what I do to the best of my ability. Would I love for a little budgie, tiel, lovie or whatever to love me and only me and spend all his time with me? Of course I would! But I would not even consider it because I know that while his would make me happy, the bird would not have the best life I can give it. I actually have a little bird like that: Codee, a handicapped GCC that came to me because of aggression but is actually the sweetest - sweetest thing you can possibly imagine - so much so that this is the only bird I allow my grandkids to handle. I had to watch my grandson yesterday and he was supposed to be in my room playing videos when I heard a door closing upstairs and, when I went to see, he was sitting in front of the computer with Codee on his shoulder! He had sneaked her out of her cage during the noon rest
And, no, of course I do not care about bonding with aviary species! For one thing, it's impossible (the most you can get is trust and friendship) and, for another, no bird lover would because all we care about is the birds wellbeing and bonding with a human is not necessary at all for this. As a matter of fact, it's usually bad for them. They get all stressed out and live super lonely lives because no human can replace a flock or a mate. How would you feel if you had been taken from your parents when you were two years old and forced to live all by yourself with giant predator aliens all your life, never having a friend or a spouse, not even somebody to speak your own language? Not too happy, I bet. But if all you want is a bird that is not afraid of you and will occasionally and on its own initiative perch on your shoulder for a couple of minutes, then you are not looking to bond with them or even tame them, you are looking for a bird that trusts you and I told you how to achieve that: patience and time. Parrots are intelligent but they do not need training to be entertained. They don't get any training in the wild, do they? Nope. They don't even get toys - all they do is eat, drink, fly, bathe and interact with their mates or flock mates. That is the life that makes them happy because that is the life they evolved to have. Not my opinion, a scientific fact.
And let me tell you a secret. All breeders claim to love their birds (if they didn't, nobody would buy from them, right? Their 'love' is just merchandising, same as a car dealer will tell you that the brand they sell is their favorite one) but none of them do because, if they did, they would not breed them for the pet trade but only give the accidental babies away to excellent homes. To a breeder, a baby bird is merchandise that needs to be moved, nothing else. Unless the breeder is somebody who does not know enough about birds to even realize that there are no excellent homes for them - in which case this person has no business breeding them at all! I have taken many deformed babies from budgie breeders when I had the rescue and have found that the greatest majority of them know very little about how to keep them the right way... they free-feed them seeds and little else, keep them at a human light schedule so they breed them all year round (which is terribly unhealthy for them!) and never even keep records or track of which one produces what (something that all good breeders do).
And yes, you will find lots of people that give advice but the trick is knowing which advice is good for the birds and which is not - something that is actually very hard for a beginner to figure out.
https://biologyboom.com/social-behavior/https://www.britannica.com/topic/animal ... in-animalshttps://www.britannica.com/topic/animal ... itorialityhttps://www.sciencedirect.com/science/a ... 7217302543https://en.wikipedia.org/wiki/Sociality