How old is the younger one? Any chance either of them could be ND? I wonder because of the way you say that other people don't seem to have the same issues, the advice in SWR didn't help, and both of them are fine individually. (Which can be the case with ND kids if you are tuned into them and can accommodate their needs, which is easier when you're 1:1).
My younger two are three years apart, DS2 has ADHD and a lot of autistic traits, and managing his relationship with DS3 is a constant thing, at least it very much was when they were 5 and 2 (and younger). Now they are 4 and 7 and it's not so full on all the time. We moved just over a year ago and one of the big things we prioritised when choosing a new house was that they could each have their own space when playing during the day - this is a non negotiable. They share a bedroom, but there are periods of the day where they absolutely need to be separate. Teaching them to recognise when they need space from each other, as a neutral thing, not a punishment to be separated when they are doing something wrong, has been incredibly helpful.
When they were younger, probably the age yours are now, there was a lot of sitting in between them when they played and sort of translating their intentions to the other one. For example DS2 would panic that toddler DS3 was going to break his tower/train track/etc as soon as he came anywhere close. So I'd narrate DS3's body language and what I could see that he wanted to do back to DS2. I would also remind DS2 that we could rebuild any parts of the track. When it came to an intricate part that would be harder to rebuild, I would stop DS3 nicely and redirect him to a part he could build himself. When DS2 did not like this, I would remind him that DS3 would get bored soon and we could rebuild it how he wanted.
It also helped a lot to actively target some of the behaviours that I found especially difficult to deal with when they are together. The two you mention - boisterousness and whingeing - am I right in thinking that part of these things being easier to deal with 1:1 is because you can essentially avoid them? With boisterousness it's easy to bundle one child outside if you don't need to rug up a reluctant other child, and the boisterousness might not build up so much if they do not have to wait around for the other's needs. With whingeing you can get them what they want almost before they have to whine or demand. So in a way it may be that 1:1 you're avoiding these things, rather than managing them, and it might make life more harmonious in general if you can actively target and address behaviours in each child which are causing difficulty when you're all together. I found this video course good - the tone is a bit annoying but the advice in it is more directive than the How To Talk type books and it does tend to work, I have found anyway.
https://www.youtube.com/playlist?list=PLh9mgdi4rNeyEGNxBvNdOVlianDYgWuc9
As a final thought, I have an older child as well who is 10 years older than DS2, so I did have the "only child" experience too. For me, personally, I prefer having two. Although there is a lot of management, I just like the experience more. I don't know what it is - I think it is the novelty (having three totally different children) and the dynamics between them (DS2/3 more so than DS1, who barely ever interacts with them). If I can have a preference for having two+, I think it's clear that having a preference for one must exist too. I know that's less socially acceptable (though one-and-done is definitely more of a thing these days) and there must be a taboo around feeling you would prefer a one child family if you already have two. But if you can embrace the fact that actually you are more suited to dealing with one child at a time and try to drop the guilt about it, it might be easier to look at arrangements where you can use childcare/playdates/divide and conquer with DH etc in order to have more time spent 1:1 with each child. I don't think this is necessarily a rejection as long as it is roughly equal. I also do things sometimes like each child has an allocation of individual screen time, and I let them have it at opposite times so I can do things with the other child.