There was a British guy called Nigel Gates who was among the better 5/10 guys in the world in the '80s, maybe into the '90s. I can't find anything specific and later he wrote a training book, which I have but also cannot find, kind of the standard :"How to Run Your Best" kind of book, and the one point he makes that I do clearly recall was that you should run twice a day even if you don't run more miles by doing it. He says you'll get the stimulus of a run twice rather than once, that if you stretch you'll get two sessions rather than one, etc.
In the '70s when Runner's World was still semi interested in running for performance they ran a long article exploring the effects of different lengths of runs. (I still have the issue but in keeping with a theme here, I don't know where. Maybe its with Gates' book.)In the article they referenced a study on British female marathon runners for factors that seemed to lead to faster races. The number one variable was frequency of runs. More were better. It was not number of miles runs, that was the number two variable.
You can find examples of successful runners who do low volume and some of them run doubles. One of the more interesting ones was a guy from Connecticut called Charlie Robbins who had won many national road championships in the '40s and '50s and remained a very good age group runner into his eighties, just before he died. He was profiled in Runner's World as well and said he did two fifteen minute runs a day. Bowerman was never an advocate of big mileage but running twice a day was a major part of his system.
I would think that at some point you are running so little that run there's no point in doubling but also suspect it's quite low. Robbins was probably doing three miles a day. If you're only going to do a mile a day would two half mile runs be the way to go? That seems a bit extreme. Lydiard once said that even a fifteen minute run was really beneficial. That's what Robbins was doing so maybe up to fifteen minutes you might as well run once but after that there are benefits to doubling. But this leads to the matter of why someone like the OP is setting 35 miles a week as a ceiling?
On the other hand, in Marty Liquori's book he argues for doing one run a day unless you're nearing national class. Otherwise, he calls running doubles "pretentious." Ron Clarke always ran twice day, an easy 5-6 miles in the morning with his wife riding alongside him on a bike. But he did not consider that run training. It was time alone with his wife. He told me that he'd have been exactly the same runner had he just run once a day.