I am certainly no expert and I'm very new at this, so the experts on this forum can definitely give you great advice.
What I can tell you... is that it's possible... that Buddy will grow out of this aggressive and biting phase. My male budgie is about 7 months old, and he has gone through and still is going through an aggressive, bitey & territorial phase. And he's a very sweet bird and was definitely a sweet and docile youngster.
This phase, sometimes referred to as the "teenager phase" or "budgie puberty" is apparently pretty normal. My budgie will bite me for a whole host of reasons (doesn't want to come out of the cage, doesn't want to go back in the cage, thinks I'm getting too close to his toy, when I'm putting his fresh kale - INTO - the cage with a clip, etc., if he's feeling grouchy or moody and wants to be by himself, if he's too excited, etc.)
At this age they're coursing with hormones and they're learning how to be adults and independent instincts come up, that are hardwired to protect their "nest" or their cage.
Also they start a molt around this age, which can make budgies uncomfortable and grouchy.
You can't take his behavior personally. It's not. He still likes you.
What I did was observe when my budgie is most likely to lunge at me and bite me and I figured out strategies to avoid that. (Mornings, evenings.... there are certain times of the day right now when he is grouchier. Or when he's with a favorite toy, he might get mad if I try to take it away. Or, sometimes he becomes territorial at his playgym...it's HIS... and he doesn't want to go back in his cage sometimes when he's on HIS playgym.
He's become a bit bossy and on some days... he'll do a bossy warning noise all day long. Those are the days i can do nothing right. I like to describe the noise as sounding like an "angry dolphin." It's actually sort of funny how bossy he is on some days. It's not every day. It's been maybe... 3 or 4 times. Today for example, he's very sweet & pleasant.
I have every confidence Buddy will grow out of this.
What I've done ... is made sure I'm more cautious of offering my hand in this growing phase. If I think my bird is a bit ornery... I offer him a wooden chopstick or a perch to step up on. That way I avoid the bites. This has worked well. If he's in an aggressive mood, he can then attack the perch or chopstick and not my hand. If he's in a good mood... he'll step up easily and then I can offer my hand from there.
He did once go maniacally crazy on my hand, attacking it with multiple bites, and hard... but I ignored it and it hasn't happened since. I think it was an exception and he was having a bad & crazy moment. I didn't take it personally.
I think that lots of toys and chewing toys can help work out some of his aggressions at this time.
It sounds like maybe you need to keep the (2) birds in two separate cages for now, and let Buddy get through this growing phase?
Again, I'm not an expert and there others on this forum that can guide you better than I can. However these are some of my experiences that I've been going through. Hope it helps. Good luck...!
p.s. -- Here's a really good sticky article in Budgie Behavior section of the forum: https://talkbudgies.com/budgie-behavi...ggressive.html