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If I'm correctly interpreting your problem, what you're looking at is a controls problem more than a urdf problem (at least in my opinion). My solution would be to write a controller for your joints that would be able to perform the correct calculations and issue the correct motor commands to preserve the movement that you desire. I don't think that it would be possible with pure urdf.

You can look here and follow the tutorial chain to get you started. I hope that this helps.