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In this paper, we examine creation of a decentralized kinematic control scheme for a composite system of two (or more) wheeled mobile manipulators that can team up to cooperatively transport a common payload. Each mobile manipulator module consists of a differentially driven wheeled mobile robot (WMR) with a mounted planar two-degree-of-freedom (d.o.f) manipulator. A composite multi-degree-of-freedom system is formed when a payload is placed at the end effectors of multiple such modules with significant advantages. However, the nonholonomic/holonomic constraints and active/passive components within the composite vehicle need careful treatment for realizing the payload transport task. Hence, we first verify that arbitrary desired end-effector motions can be accommodated, within the feasible motion distributions of the articulations and the wheeled base. Then, we develop motion-plans by which this desired end-effector motion could be actively realized, using only the limited active motion-distribution of the differentially driven wheels. Finally, we deploy this in the form of a two-level hierarchical control framework, with an upper-level planning of the steerable active vector-fields and a lower-level posture stabilization control of the individual WMRs. Preliminary experimental results from the decentralized-control implementation for a two-module composite vehicle are also presented.