Like,
Multi motor set up, You can select from more than one output "profile" from a single VFD, the motors need not be the same.
Most "better quality" VFD's are recognized as safety devices. Check the specs.
I would say that if the drive required is less than 15kw, the cost of a second drive might off set the control installation costs for a single VFD split between two pumps. Bigger drives start running into real money;
I don't know why you need to switch between two motors on a "runs sometime" application, but there must be a reason. ;-)