I'll toss in my 2 cents
We added a second male, English Budgie in February. Bao is an adult male, around a year old. He was one of several English Budgies owned by an elderly finch breeder who was retiring due to health reasons. He was not hand raised or fed, but has tamed down nicely. He's still a bit skittish when you put your hand in the cage, but steps up right away... never bites, and is sooo sweet. Bubbles-our female; we've had since she was around 6-8 weeks old, she was hand fed and around people her entire life. Bubbles and I are very bonded, she's regurgitated to me, preens my face, always wants to eat my food/drink, and is happy to sit on my shoulder for hours. Bao on the other hand, will fly off my shoulder after a bit, refuses to take ANY food from my hand, and would prefer to hang out alone or with Bubbles in the cage. Bubbles is almost 2 years old now, but her bond with me hasn't changed much since we added Bao. She's my sassy poof poof girl. Bao... well... he's cute and mellow. Bao HAS recently fallen in love with my fingernails however... head bobbing and regurgitating to my thumb especially. So... progress??? (no he's not masturbating.... yet
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I think it depends on the bird AND your bond with your first bird before you add another. I wasn't going to add a second bird, but I ended up going back to work full time and Bao needed a home... so it was a win-win. I have NEVER seen them preen each other. Bao feeds Bubbles, but Bubbles NEVER reciprocates. Bao will go out of his way to go around her, with a healthy distance, if he wants to go somewhere in the cage and she's in the way. I've seen him hop from a perch, to the side of the cage, to another perch to go up and around her. So, I honestly don't known how well bonded they are. I've never seen them actually fight down at the bottom of the cage, nor have I ever seen anything that worried me. Bubbles is just a diva, bossy hen! I don't want eggs/babies either... just FYI.
Also, in regards to the AC issue, I recently bought a ceramic heat bulb (in the lower wattage range) and put it in a metal utility lamp above the cage. It's the kind for reptiles, and my Vet said it would be fine to make a "warm spot" in the cage. Birds can move away if they want. I turn it on when our house AC is on, in case they get cold. Bubbles has also always had one of those small, flat heaters that is made for birds, that screws to the side of the cage. She likes to rub her head on it.