The 656 does have two pumps mounted piggy back. There is a difference though from the older 560 for example. On the 560 the main pump feed the aux system and the smaller piggy back pump is strictly for the power steering.
On the 656 the larger main pump first goes to flow divider for steering and balance goes to aux valves. The smaller piggy pump is for hitch but if hitch is not raising the flow from hitch pump joins the flow from other pump to go to the aux valves. Separate relief valves for each system also. So, first thing to check is relief valve for aux valves located in the rear of aux valve block. If sounds like sucking air check return hose inside rear of tractor through 3rd arm link plate.