string[] mods = System.IO.File.ReadAllLines("mods.txt");
string[] mods_temp = System.IO.File.ReadAllLines("mods_temp.txt");

var intersect = mods.Intersect(mods_temp).ToArray();

mods = mods.Except(intersect).ToArray();
mods_temp = mods_temp.Except(intersect).ToArray();

var mods2 = mods.Except(mods_temp);
var mods_temp2 = mods_temp.Except(mods);