Typical sunglasses absorb 90% of light. Therefore two sunglasses on top another would absorb 99% and let 1% through. Is the remaining light enough to blind you when looking directly at the sun? I assumed yes, but I might be wrong. I'll have to find a second pair of shades to test.

Can you look directly at the sun with a Welding Filter shade 5?

Not continuously for hours, that takes about 14 or 15. But it isn't dazzling.