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I think a safe jack is one of the most used and most important tools in any garage. Trying to put more than one wood block or plank on a jack sounds like a good way to get hurt or damage your car. I'd suggest that you spend the $ for a proper jack.
+1 to that.
I bought a craftsman series 3 ton jack which came with two jack stands for 99$. This thing is a beast and fits under my lowered mustang.
Last thing you need is to dump your car in the garage.