Any chance that this could return an error: I think it will be a common mistake
(the error is entering the same submodel twice instead of two distinct models (in this case modelDZ and modelMZ).
the name clash maybe should throw an error
"did you really want to enter "modelMZ" and then immediately overwrite it with another copy of the same thing "modelMZ"?
model = mxModel("ACE",
modelShare, modelMZ, modelMZ,
mxAlgebra(modelMZ.objective + modelDZ.objective, name="twin"),