US route duplexes I can think of:
US 1 and 23 between Jacksonville, FL and north of Alma, GA - 106 miles
US 14 and 16 between Ucross, WY and Moorcroft, WY - 111 miles
US 20 and 26 between Vale, OR and Mountain Home, ID - 113 miles
US 19 and 98 between Perry, FL and Chassahowitzka, FL - 127 miles
US 20 and 26 between Shoshoni, WY and east of Douglas, WY - 160 miles
US 85 and 87 (not sure how well this one is signed) from Raton, NM to I-25 Exit 185 near Castle Rock, CO - 194 miles
A couple of historic ones:
US 60 and 70 between Beaumont, CA and Globe, AZ - 400 miles
US 6 and 50 between Ely, NV and Grand Junction, CO - 470 miles
US 70 and 99 between Los Angeles, CA and Indio, CA (also with I-10) - 127 miles
US 40 and 95 from west of Lovelock, NV to Winnemucca, NV - 96 miles