The original question is whether doubling helps you recover better. You could answer that for yourself experientially. Personally, I found that doing a short, easy run before my main run often made the main run feel a lot better. Later in my life things changed so that I needed to do the main session early in the day and the short, easy, run later. That did not seem to make the next day's main run seem easier as had happened when I did the main session second. But your results may differ.
It's not too hard to find examples of people whose running improved a lot when they started doubling. One that comes to mind quickly is Barry Magee who started off doing the usual Lydiard 100 mile weeks on one run. He told me he got to national class on that system but became an international class runner when he added a short second run and got his total miles to 120-130. It is possible that it was the "extra" 20-30 miles that got him there and it would have happened for him had he just added another 3-4 miles to those main runs. However, going out for a 17-20 mile run every day can get wearing both mentally and physically. But I know stories of lesser known runners who switched to doubles who did not increase their overall mileage and made significant improvements.
The problem with the question you raise in your last paragraph is that a runner who goes from doubles to singles will have the benefit of the fitness gained from time spent doubling. I made the switch you mention in 1978 and did not slow at all for a couple years. I even eked out a few small PRs. BUT in 1973 I made the switch from singles to doubles (with big mileage increases) and made huge breakthroughs.
In the late 70s, when Runner's World was still interested in serious running, they referenced a study done on female British marathoners looking to see which training variable was the best determiner of racing success. It turned out to be number of sessions done in the preparation period. That's NUMBER of sessions, not overall mileage. Mileage was the #2 determiner.
And no, I have no idea how to track down that study after all these decades or why they chose female British marathoners.
You can find many examples of people who ran well on just one run a day. Ron Clarke almost always ran twice but told me that he thought he'd have been exactly the same runner on one run as he'd been on two. For him, the 5-6 mile AM run was often done with his wife riding along on a bike and they saw the run as time to catch up with each other without distractions. On the other hand, he did two runs and was just speculating that he'd have been as good had he just done one.
Most really successful runners have doubled and that's been the case at least since the end of WWII. You're faced with a choice of doing what most successful runners have done or thinking you've figured out something better and trying it out. In either case, do not fall into the trap of thinking that your "physiology" is different from elite runners. It's not that different and the principles that work for you are the same ones working for them. The application has to be adjusted. At some point you cannot think or read yourself to the best way to train. You have to try things yourself and see what happens.