First and foremost, sorry if this thread is in the wrong topic; I didn't know where to open this thread.
TL;DR - I've had a young budgie, I brought home an adult one, now my younger budgie is sick.
The long story:
I have two budgies, "Maviş" and "Budgie". I've had Maviş for 4 months and she was my grandmothers bird before that. They picked her from a local pet shop. I estimate that she is around 7-8 months old. She was wild at first, but she got used to me and she would even fly to my hand when I put him on his cage.
Later, I decided that Maviş was lonely and I wanted to introduce her to another budgie, so I picked "Budgie" from a local avian pet shop. The guy said Budgie was around 1.5 years old. I brought him home and put his cage next to Maviş's cage, so that they would get used to each other. I didn't know to quarantine the newcomer bird.
Two or three days later, I let both birds out of their cage so they would meet outside. I removed Maviş's cage since it was the small one, and redecorated Budgie's cage for both birds. After a while, both birds started using the cage; they could find food and water easily, they were going inside to sleep and come out when it was day, etc.
However, as days passed, Maviş started sleeping a lot. She would sleep with his head hanging down low (this started happening before I brought Budgie home). I began to worry that she was sick; but the guy at the local pet shop said that every bird sleeps differently, so it could be normal.
I took Maviş to the vet when she looked like she had diarrhea, and they gave vitamin drops and an antibiotic of some sort. I put the meds in their water, but I didn't separate the birds from each other. As days passed on, she looked like she was okay again, and her diarrhea looked like it was gone.
A week passed, I was out of town. When I came back, Maviş looked healthy but she woke up in the middle of the night and vomited seeds. She looked like she was weak and I could easily pick her up with my hand (she would usually try to bite me and free herself). She wouldn't mind me touching her, she didn't ate fruit bits when I tried to feed her and didn't drink any water. Her wings looked like they were hanging from her side, and she couldn't pull them up her body. I took her to the vet immediately, they gave her some medicine and exposed her to some oxygen. They told me to continue putting vitamin drops in her water, and I went back home.
I had a little talk with the vet, and he said that they had a bird brought to them with the same reason. The story was the same, the woman/man had her/his budgie for over 6 years, but as soon as she/he brought home the companion, the older bird started to look sick. Vet said that maybe Maviş was sad because of the newcomer, and it made her sick. He didn't sound sure, and it wasn't more than just a guess.
I have to make a decision now, and I need help. I'll be moving out of town in two weeks, to a new place, and I plan to bring my birds with me. However, I don't know if I should bring both of them or only Maviş. They don't get along very well, so the theory of Maviş being sad doesn't seem like nonsense to me. However, I will be having to give Budgie back to the same pet shop, and I don't want to make him spend his life inside little a cage. I'm hoping all of this is because I didn't quarantined Budgie when I brought him home, and I'm hoping both of them will get healthy so I won't have to give up on one of them.
Sorry for the long post.