if the rigid body is fixed.
You can't have the same rigid bodies in 2 worlds. A rigid body has at the very least a pointer back to the world it is in and will assert if you try to add it to a different world. That being said, a rigid body by itself has little memory overhead (about 640 bytes). Most of the memory is going to come from the hkpShape associated with it. You should be able to share the shape data however since that is pretty agnostic to what is using it.