How To Define Special Arm Mechanism in URDF?

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Good suggestion. However, the mimic tag would work if you just wanted to slave one joint to another. I still want to have some independent control over the second segment. I forgot to say that both motors will be located at the arm's base and will operate both segments independently via pulleys.

I think the mimic tag would work if OFFSET parameter could be tied to second segment's joint angle. e.g., first arm segment moves X deg relative to the base and second arm segment moves X - Y deg relative to the first arm segment where X and Y are the command joint angles from the motion planner.

Is it possible to make the MIMIC OFFSET parameter a variable? But I think the real solution is do something within TF so that these transformations are modeled correctly and the planning engine has full sight of them.