I want to have an object that stay at one side of an actor, and so its physics don't need to be computed, and the prediction can be done with the actor itself.
How do I do this properly ?
The scenario you are describing probably consists of only one game object, which has two rendering resources. At all times there is only one physical entity. The dimensions of the physical entity may need to be enlarged to represent both resources.
Take for example, a pirate and his parrot. The parrot is constantly hovering around the pirate, wherever the pirate goes. In this case, the correct approach is to use only a single game object, which represents both the pirate and the parrot. The pirate has one game object, but this game object has two render resources which are associated with it. Those two resources could be two sprites: one for the pirate and one for his parrot.
Note: There actually is a way to disable client-side prediction for a game object. Any class which derives from GameObject, including DynamicObject and PhysicalObject, can override the following two functions to return zero instead of null. This will disable all bending.
get bendingMultiple() { return null; }
get bendingVelocityMultiple() { return null; }