Plot tree versus another tree


I simply want to plot a tree versus another one.

I could do something like :
TTree* ntup1 = (TTree*)f->Get(“CollectionTree”);
histo1 = new TH1F(“h1”,“T2Calo Eta”, 1000, 1, 1.1);

when both x1 and x2 were in the same “CollectionTree”.

i now want to plot the same thing
but x1 is gotten from one Tree
TTree* ntup1 = (TTree*)f->Get(“CollectionTree”);

and x2 is in another tree!
TTree* ntup2 = (TTree*)f->Get(“POOLContainer_TrigEMCluster”);

how can i plot x1-x2 versus x2, when x1 is accessible via ntup1, and x2 via ntup2

Thank you very much

Declare ntup2 to be a friend of ntup1. Seee TTree::AddFriend … :AddFriend