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As stated in the Subject section, I plan to add an external object to two bodies in a model. To be more specific, the model is holding a rod with both hands, hence the rod should be attached to both hands with two custom joints, I presume? So far I manage to attach one hand to the rod, but I am unable to attach the rod to another hand, can anyone share with me how to do that?

Please see attached a conference abstract that explains different options and their effects in more detail.

John, this is something I have been exploring using different implementations for modeling backpack load sharing:
https://www.researchgate.net/publicatio ... d_carriage

There is a good discussion of this issue in the SimBody theory manual https://simtk.org/docman/view.php/47/23 ... Manual.pdf, which should help you.

If you want to define a joint at each hand, you need to break the kinematic chain by splitting the rod into two bodies then constraining them together with a weld or bushing. You still need to be careful with this implementation because you can end up coupling the motion between the joints via the weld based on which degrees of freedom are mobilized. If you don't need a joint at each hand, you can attach the rod using bushings and maybe some other force elements. See what works best for you.