I have been actually looking for a Blue-Crowned Conure myself for a few months to add to my flock, I've always wanted one, they are a wonderful species of Conure, the are intelligent, funny, cuddly, loving Conures...I have worked with a few over the years at the Avian Rescue I have worked at for a long time, and I've loved every single Blue Crown we've had...I'm actually having trouble finding one, either an adult that needs a home or a hand-raised baby...That's really the only thing holding me back right now...
As far as your Green Cheek goes (I also have a Green Cheek, a male Yellow-Sided who is turning 3 next month), there is just no guarantee that he will get along with any other bird that you add to YOUR FLOCK, since he is bonded to you and the new Blue-Crown will also be your bird and bonded to you, or at least that's the plan. Green Cheeks are very territorial, jealous little birds, and my Bowie is absolutely without a doubt the most-jealous of my 4 (in addition to Bowie I have a Senegal Parrot, a Quaker Parrot, and a Cockatiel), and Bowie will definitely be the one who has the most trouble with any other bird I add to the flock, though again you just don't know what is going to happen with them, whether your Green Cheek will get along with the new Blue Crown, OR vice-versa, you don't know how the new Blue-Crown will get along with your Green Cheek.
The bottom-line here is that you have to be 100% certain that you are going to be able to have enough time in your daily schedule/routines that you will be able to spend enough time with each bird separately , if need be, as well as be able to set-up your home where both of their cages are located in the main-room of your home, where they can both be present but separated across the room, and where you'll be able to provide them both with enough out-of-cage-time as well as with enough one-on-one time with you that they will both be happy, just in-case it turns out that they cannot be out of their cages together in the same room with each other...If you're not sure that you can do this, then I don't suggest that you get any other bird, regardless of the species, gender, age, etc. And that's because they may love each other and bond-closely with each other, they may like each other enough to hang-out together but not really bond with each other, they might just simply "tolerate" each other but not like each other much, they might really dislike each other and just need to be kept separated (especially when around you), or they might have a very strong dislike for each other, probably based on the jealousy of your Green Cheek, at least in the beginning, and may not be able to be out of their cages at the same time AT ALL without being aggressive and violent with each other...Or any combination of all of these...You just can't guess or know, and you must be able to provide them both with enough one-on-one time with you, separately and at different times if need-be, otherwise it's just not fair to either bird...
My daily schedule is nuts due to the different flock-dynamics between my 4 guys (and the Ringneck Dove that is bonded to my Cockatiel but who the other 3 hate, lol), so we have a set routine/schedule that allows each of them equal time with me based on who gets along with who...And it works well for us, actually very well. And you learn what you can and cannot do while you have them all together at once, you learn to give each of them there own "territories" to sit on while you're with all of them at the same time and they're out of their cages, and you figure it out...But you have to have the time to be able to do so...
And as mentioned above, your Green Cheek has yet to go through puberty, so his personality may or may not totally change afterwards, and his relationship with you may become horribly territorial and he may become horribly protective and jealous over anyone or anything that go nears you, you just don't know. So I'd wait until your Green Cheek is at least a year and a half old so you can see what he is going to be like with regard to you and others in your home before you bring home another bird to be your companion too, because your Green Cheek might start biting/attacking everyone who goes anywhere near you, even your family, or he might transfer is bond to someone else in your house, who knows what will happen once he becomes hormonal...