[Moveit]Howto make two joints always have identical position

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I have a robot arm where I have two revolute joints (one driven and one forced) which will always have the same position (angle in my case) due the mechanical constrains of my robot.

What is the best way to implement a Moveit configuration for this kind of situation?

My first idea was threat both joints as normal Joints, and send fake axis information for the forced one from my Robot controller. This solution should work, but the motion planned would have to do extra calculation for this joint too.

I am unsure if the virtual joints could be used for this purpose.