Adding Likelihoods depending on RooArgList order

Hi,
I have a problem of adding different likelihoods. If I import 3 (or 2) RooRealNll-objects from different workspaces in different saved ROOT files and I add them together using

# python
NLL_list = RooArgList()
NLL_list.add(NLL1)
NLL_list.add(NLL2)
NLL_list.add(NLL3)

Combined_NLL = RooAddition('nllCombi','combined NLL', NLL_list)

the resulting combined likelihood curve depends on the order in which I add the NLLs (see attached screenshot and full python-testscript without workspace Objects).
Also, the absolute value of the combined NLL curve doesn’t make full sense, as it is higher then you would expect from the sum of the 3 (expect ~ -11000).
All in all I cannot trust the resulting likelihood curve at all. Contrary to an example in a RooFit talk ([indico.in2p3.fr/materialDisplay.py?contribId=15&materialId=slides&confId=750]School of Statistics 2008[/url]) I had to give each NLL-object an individual name first, otherwise only the curve added first is added to itself.

Does anyone have a solution on how to reliably add likelihood functions in RooFit?
combine_likelihoods.py (2.97 KB)
Comb NLL ID401 ID3 ID6.eps (17.2 KB)
Comb NLL ID6 ID401 ID3.eps (17 KB)
Comb NLL ID3 ID6 ID401.eps (17.1 KB)