This paper addresses the problem of tracking control of multiple hydraulic manipulators handling a rigid object. We propose a controller that allows two or more hydraulically-actuated robots cooperatively, to move a rigid object along a desired trajectory while sharing the load and maintaining an acceptable internal force on the object. The parasitic effect of friction in the actuators, as well as parametric uncertainties in the manipulators' dynamics, hydraulic functions and the payload, are compensated through augmentation of the controller by a set of online updating laws. Additionally, an adaptive observer is used along with the controller to avoid the requirement of acceleration feedback, which could be difficult to obtain in practice. Only measurements of joint positions, joint velocities and hydraulic line pressures are required for the implementation of the controller. The tracking error of the control system is proven to converge to zero while the internal force remains bounded. It is further proven that the internal force can be set as close as possible to the desired value through the addition of a simple force feedback loop with an adjustable gain. Both simulation and experimental studies are conducted to clearly illustrate the effectiveness of the developed controller for various tracking tasks. The simulations are carried out using a complete nonlinear model of two multiple-degree-of-freedom industrial hydraulic manipulators handling a rigid object. Experiments are carried out on an available fully-instrumented electro-hydraulic system.