Bump starting, checking of correct rotational direction should be done un-couple as mention by Abeltion. In any case there is a possibility for the impeller locknut to work loosen even reverse turning for a couple of second.
Secondly for some high speed machine equipped with integrated oil pump...
Stand by pump normally are auto cut-in. Majority of which are base on low or low-low pressure. Of course minority are base on level and capacity.
It is definetely not adviseable manually or hand turning the stand by unit in any case when the main pump is in operation due to safety reason, which...
Stand by pump, often refer to vital equipment with continuous service. It does not normally provide spare capacity, but more toward availability.
Pumps with intermetent service and having N+1 or N+2 should considered as spare pump because they provide spare capacity.
To generate swapping...
Something to look at to avoid cavitation from air trap & high turbulence. You have a reducer tie to your pump suction, is it a concentric reducer? What is the reduction size?