Plumbing of one pump which utilizes 2 dumps is outlined in the diagram below. This is typically the way one pump is plumbed in a two pump system. The second pump would be setup the same way.
It is important to use fittings, hoses and checks that are designed to withstand the high pressures of a hydraulic system. Normally, they are rated at least above 3,000 psi. On the low pressure side, it is okay to use lower ratings. For example, brass slow down valves (needle valves) will work fine on the low side.
The arrows indicate the direction of flow. When the motor is activated, it turns the pump head which forces the hydraulic oil through the high pressure side. Oil passes through the check valves and out through the dumps to the cylinders which then lift the car. On activating the dump (which is normally closed) it opens the valve and allows the oil to return from the cylinders, follow the return lines through the slow down valve and ends up back in the reservoir tank. The check valves keep the oil from returning to the pressure side of the block.