This is from a member of tge forum from a few years ago.
" The best advice I can give you is how we 'teach' the birds at the shelter to at the very least, tolerate each other. Birds will always have opinions in who they like best, but you can 'teach' them to get along. (I am putting teach in inverted commas here, as you can't really teach them, you can associate treats with being around that particular bird. (; )
Try to limit inter-bird interaction to around 15-20 minutes. During this time, feed many treats to both birds, and repeat this for around about two weeks, or however long it takes for the birds to associate treats with each other. (this doesn't have to be every day).
After you're confident that the birds won't hurt or attack each other, try putting the treat in a food bowl where the birds have to ''share' the treats. If one bird lunges or shows annoyance, move them to another play stand/perch. Hopefully at least two of your birds will do this, therefore showing the troublesome bird that getting along = treats. Try gain the next day.
This process is all about repetition and slow introduction. I would say that here at the shelter, this method works around about 75 - 85% of the time. If it doesn't work for you (as it all depends on your birds) there are certainly other ways, but this is probably the easiest and best for the birds. Hope that all your birds get along nicely and become a big, happy flock!"