Multiple parents. Is it possible? SDF format.

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I want to implement a simple gearbox for my rocker-bogie rover replica. In order to implement the suspension I intend to use a simple gearbox. But it turns out that it is not possible to make it without having a link with two joints and two parents. The first joint is revolution joint and the second one is gearbox joint. The question is: "Is it possible for one link to have two parent in joins?" I use sdf format to describe my world and my model.