"two similar gauge symmetry group produce two different local transformations. Can you give an example of how it do it (if this is valid at all)?"
Imagine that there is regular matter that has regular electromagnetic charge and weak isospin.
Imagine that dark matter is composed of something called "mirror matter" and that mirror matter has mirror charge and mirror isospin that is mediated by the futon, the V boson and the Y boson, which follow precisely the same laws as and have the same properties as the photon, W boson and Z boson respectively do with respect to the corresponding properties of mirror matter. Imagine further that futons, V bosons and Y bosons never couple to regular matter, while photons, W bosons and Z bosons never couple to mirror matter.
Voila, I have just completely described how two identical gauge symmetry groups of the SU(2)xU(1) type produced two different local transformations.
Another fairly common proposal would be to propose something along the line of a theory in which quarks are themselves composed of three "preons" bound together by an exchange of "supergluons" that come in three "tints" which we might call black, gray and white, that we would call the "superstrong force" that is identical in all respects to the SU(3) gauge group of QCD except that the coupling constant of the superstrong force was much larger than the coupling constant of QCD. Not very original, I agree. But, honestly, how original is nature in always giving us gauge boson mediated gauge group forces that propogate bosons in essentially the same way the QED does?