If i understand your question corectly, and you have several .mat files each one with a varriable T and a varriable output,
you can load them like that:
data(1) = load('D:/pendrive/coulomb_drag/Temperature/matlab_program/drag_Copy_dd/taudrag.mat');
data(2) = load('D:/pendrive/coulomb_drag/Temperature/matlab_program/drag_Copy_nn/taudrag.mat');
that syntax returns a struct with fields for each varriable saved in that .mat file.
that would only work if you have the exact same varriable names saved in both files, otherwise you can simply load them into two different varriables
for i = 1:numel(data)
dataStruct = data(i);
plot(dataStruct.T, dataStruct.output);
hold on;
end
