If you have one column in your files, try the macro: Read txt file and draw histogram
and replace:
t->ReadFile(“a.txt”, “v/D”);
with two lines:
t->ReadFile(“FirstFile.txt”, “v/D”);
t->ReadFile(“SecondFile.txt”);
If you have two columns in your files, try: TTree *t = new TTree("t", "a tree");
t->ReadFile("FirstFile.txt", "x/D:y");
t->ReadFile("SecondFile.txt");
t->Draw("y:x");