'**SUMMARY**
stack two filters of same size. channels don\'t matter.
**PARAMETERS**
* *flt1* - second filter to be stacked
**RETURNS**
DFT filter'

def _stackFilters(self, flt1):

    if isinstance(self._numpy, type(None)):
        return flt1
    if (not (self.size() == flt1.size())):
        warnings.warn('All    the    filters    must    be    of    same    size')
        return None
    numpyflt = self._numpy
    numpyflt1 = flt1._numpy
    flt = np.dstack((numpyflt, numpyflt1))
    stackedfilter = DFT(size=self.size(), numpyarray=flt, channels=(self.channels + flt1.channels), type=self._type, frequency=self._freqpass)
    return stackedfilter
