Why use a two stage pump to power a hyd motor.
You want constant flow to a hyd motor, and the volume of the fluid should be constant, or you will not develop the blade tip speed for efficient cutting
There are plenty of good single stage hyd pumps to provide the pressure and volume of fluid to run a hyd driven deck.
You just need to figure the HP required to power the pump that will feed the hyd motor., and the HP developed by the hyd motor will be about 85% less.
The below deck is using about a 3 cu in hyd motor.