How to join 2 rigidbody in run time ?????

I have 2 rigidbody and i want to join them using fixed join in run time...please help how to do it ?????

hmm it is working nut i have another problem...i have 2 cars and i drive with both of them...but i want to in one time of run time to join first second car to first....i use this but when i want to play i can run 1. car but second will not move because hint join :S