I understand the impulse to convert to a dual master cylinder. Redundancy is your friend in safety systems.
That said, I have been running a single master cylinder for over 20 years in San Francisco and greater Bay Area traffic and it only failed once. That failure was right after I purchased it. I had a bad rear wheel cylinder. I purchased a new one from NAPA and put it in. About a month after that it blew.
When I took it apart I could see that the seal had a manufacturing defect. After that I always inspect all seals in all brake items even if new.
In the big Desoto I have a remote fill container up on the firewall. The end plug on the cylinders will screw into the top and you can just run a line up to the firewall. I use a white racer one so that I can just look and see how the brake level is doing. I use modern steel braided flex lines in place of the rubber. I new NAPA line got a bulge in it after about 14 months. So much for quality.
My single master cylinder was sleeved with brass, but the man who did it has retired. You can get them done with stainless as well.
If you rebuild everything carefully and keep an eye on it, the single unit system works fine. I think the real reason they went to a dual system was the lack of proper maintenance on thousands of peoples part that of course then lead to bad accidents.
James