As a young man i installed a lot of one pipe systems. They are not perfect but at the time there where less options. The biggest issue was the length of 1/2" pipe work to the rads from the 3\4" loop, if long circulation would not take place through rad. (when fitting says rad in a bay window it often caused problems as it was easy to use long runs of 1'2" than remove boards and cut joists in the restricted bay area for the 3\4" loop) It is only the frictional loss on the loop between the tees too the rad that promotes circulation through the rad. Assorts of tricks were used to get paid not least of all restricting the 3/4" loop between tees to rad with a set of mole grips (squeeze grips, feel rad, squeeze grips feel rad and so on. One plumber was reported for the following bay window rad luke warm, on visit the plumber knew the owner would be home at 4-30pm so he move a setter out the way and warmed the rad using a blow lamp, quickly putting the settee back as soon as he heard the door go. Happy punter, nice cheque. I think it was the start of giving plumbers a bad name.
As to your situation (other than selling you a blow lamp) i would turn the two rads you are proposing on to a two pipe system. Tee into both 3/4" flow and return with a 1/2" branch, split both ways and feed rads. (Almost like a bypass loop, which were not uncommon in the day) you would then have to restrict dramatically with the lock shield valves in order to maintain the existing system.
