// brun
//However (and I suspect that this is your case), if you want to compare
//2 Trees with the same number of entries and entry I of Tree 1 versus entry I
//of Tree 2, you can use Tree friends in a very simple way. Example
//Suppose 2 Trees named "T" in file1.root and file2.root

TFile f1("file1.root");
TTree *T = (TTree*)f1.Get("T");
T->AddFriend("T2=T","file2.root");
T->Draw("x:T2.x")

//The last command above will produce a scatter plot of variable "x" of "T"
//in file1.root versus variable "x" of "T" in file2.root.
//Note that "T" in file2.root has been given an alias "T2".
//For more details, see TTree::AddFriend
//  http://root.cern.ch/root/htmldoc/src/TTree.cxx.html#TTree:AddFriend
//See also $ROOTSYS/tutorials/tree3.C