Most of what I’ve read seems to recommend a 2nd (or even 3rd) carboy for the 2 years of aging.
However, I’m wondering if there is any downside to just bottle conditioning right out of the primary (or brief secondary) for the 2-year aging process. This way I wouldn’t tie up a carboy for 2 years.
I really think you can’t attain the same amount of sourness in a bottle that you can aging in a carboy. The first reason is because these sour flavors we are trying to produce develop better in the presence of oxygen, and there is very little to no flow of oxygen into the the capped bottle. Secondly, If you want to add anything, such as fruit or oak cubes to your sour beer, it can’t be done in a bottle. Last of all, sour beers are so unpredictable that the sourness produced might taste better blended. If you have your beer in a dedicated glass carboy you can periodically sample the sourness level, maybe even deciding it might be worth blending your sour with a different more younger sour beer.