Opening two .root file in the same canvas

ROOT
16

These file contains Canvases not histrograms … You are mixing up many diferents things…

root [0] TFile f("essai1_2.root")
root [1] f.ls()
TFile**		essai1_2.root	
 TFile*		essai1_2.root	
  KEY: TCanvas	Canvas 1;1	Canvas 1
root [2] TCanvas *c = f.Get("Canvas 1");
root [3] c.Draw()