Driving both oil pumps from the same shaft - the main and stand-by - evidently enables a single failure to cause complete loss of oil pressure.
True, but the risk of that shaft detaching from the upper part of the main gearbox drive shaft (which is a single point of failure anyway) may have been analysed as being lower than that of any alternative design.
(There’s a gearbox diagram on the G‑REDL accident thread
here.)