obito's eyes doesn't go blind due to the healing effect of the wood style lifeforce implanted in him. Kakashi died while using the mangekyou during his fight with pain.
Doubling 2×8 floor joists can extend the span around 25 percent with other factors identical. As an example, this means that a single floor joist with a span of 9' would have a span of 11' 2” when doubled. You can even triple the 2×8 floor joists for an even greater span.
The weight that can be supported by a double or triple wood headerSpan (feet)Weight Supported (lb)2 x 2" x 6"3 x 2" x 8"42250585061680469083990
A horizontal 2×6 plank on edge is generally rated for 50psf (10psf dead load + 40psf live load) or 53 pounds per linear foot. So, a 2×6 spanning 8 feet should be able to support (8×53) 424 pounds along its length. A 2×6 on flat will carry about 4 pounds per linear foot or around 7% of one on edge.