Twin brothers stand in front of a farmhouse—first in 1975, then again in 1995. In the span of 20 years, they grow from middle-aged to old, becoming sun-weathered and wind-beaten. Another diptych shows Tim and Tom Kehoe, also twins, posed before a barn and flanked by Holsteins. In the first image, they are boys; in the second, men. Their outfits change, haircuts shorten and lines deepen, but their demeanor is unwavering. Then there’s the portrait…