TF1: use subfunctions with equal parameters


I am trying to make a function describing the response of a photomultiplier tube, for fitting to measurements. As this is quite an extensive function, I would like to use “subfunctions”. What I mean is the following:

  TF1 *f1 = new TF1("f1","[0]*TMath::Exp([1]*x)");
  TF1 *PMTresp = new TF1("PMTresp","[0]+f1");

However, the parameter [0] in PMTresp should be the same as [0] in f1. I noticed that via the code above ROOT will assign new parameters [1] and [2] to f1.
Is there a way to tell ROOT? Or can I afterwards say that [0] and [1] are the same parameter?

Thanks in advance!