When under a car that has to be up on both ends I use a 10" wide rim and set the tires in the rim. Jackstands on the other end and it is ROCK SOLID. No shake, wiggle or anything. Car feels as stable as sitting in the ground.

This came about as a suggestion from my wife as a car that was on 4 stands and started to fall as we were standing next to it. Fortunately I was able to stop it and hold it while she grabbed a floor jack.