[Arran]

Thanks for providing the schematic link for the radio you are servicing. I took a quick look at it. Without going into detail of how electrolytic caps work in "parallel" and (opposite) "series" connections, just do this... Replace both,.. the 2 "piggybacked electrolytics" in your set with 1 ( 20 uf@ 350 volts). Replace the other with the same value. you will be good. In short to answer your question, those 2 caps installed "piggybacked" before in "parrallel" were to meet the required voltage ratings required when they were installed previously. The tech used what he had at the time in his shop. Observe polarity when you install any new electrolytics, and Best of luck with all your restorations!

Sorry but I beg to differ, the connected them in parallel to increase the capacity not the voltage rating, they put two 10 mf caps in parallel to act as a 20 mf filter. If they had connected them in series they would have increased the voltage rating to 1000 vdc since they were both 500 volt rated caps but halfed the capacity to 5 mfd.
Regards
Arran