Drive a Rigidbody to a rigid body from another simulation

[Basroil]

Those environments don't look all that complex, and higher simulation rate with sleeping of unused objects just sounds a whole lot simpler than running parallel computations for objects that really need to be calculated together (character and object collisions affecting both bodies). The article listed at that site uses an entirely different concept to what you are proposing, and you might as well make your bullet character kinematic and just use collision data from there according to that article.