Can I use an iPad Air (iPad 5) screen protector on my iPad Air 2 (iPad 6)
Yes, they are the same width and height. You can check the measurements on Apple's website. They're the same aside from the depth which has nothing to do with the screen protector.

          iPad Air 2              iPad Air
Height    9.4 inches (240 mm)     9.4 inches (240 mm)
Width     6.6 inches (169.5 mm)   6.6 inches (169.5 mm)
Depth     0.24 inch (6.1 mm)      0.29 inch (7.5 mm)