How to solve design concentric joint mechanism

A double concentric joint is one in which 2 rotary motions are transmitted simultaneously and
independently along the same axis. Two motors are generally used at one end and the motion is
transferred to the other end where arms or gears can be used to transfer the motion further.
In such a mechanism the motion of the 2 shafts needs to be independent with low friction
between them. Naturally one of the best ways of doing this is to use bearings and bearing housing to
prevent relative friction.
In addition, the whole mechanism needs to be hollow so as to allow cables to pass through
further.
Design such a concentric joint mechanism, use bearings, bearing housing, fasteners, etc.
whatever Is required for the design. The components of this design (except for standard parts like
bearings, bolts, etc.) will be machined or made through sheet metal Assume the outer shaft is 16MM
and assume other dimensions