Stacking spacers?

I really want to try getting some really tight fitment. I have a 20mm spacer on my wheels right now. I had got four of them because I was gonna put 20mm spacers all around the whole car, but the fronts were rubbing too much. So now I have just the two 20mm spacers in the rear, and I'm wondering if it's safe to stack the two 20mm spacers to make it a 40mm spacer. The first picture is with the stack on, and the second picture is with the stack off.