Running two pumps off a truck PTO usually means bolting two pump bodies together with an adaptor plate, which adds length, alignment problems and two service points. A tandem pump puts both gear sets in one housing on one PTO flange, so the installation is shorter, stiffer and cheaper to plumb.

Why 56 and 26 cc/rev as a pair instead of two identical pumps?

Because the circuits on a truck crane are not symmetric. The boom winch wants big flow for a fast lift - the 56 cc/rev section delivers that. The outriggers and the auxiliary tools want a smaller, steadier flow - the 26 cc/rev section covers that without oversizing the circuit. The DIN flange matches the truck PTO standard, so the pump bolts on the same way regardless of chassis brand, and the fixed-displacement design means there is no variable mechanism to wear out on a vehicle that spends its life outside.