a question about filter

I am designing a six-pole filter based on rectangular cavity coupled directly by aperture on the broad face.First I get the dimension of the rectangular cavity according to the center frequency f0 of the filter.Then according to the coupling theory,I compute the two eigen fruquency(f1 and f2) of the two coupling rectangular cavities coupled by the aperture on the broad face. And both f1 and f2 are bigger than f0,which leads to an offset of the synthesised filter f0. How can I let f2>f0>f1 and f0=0.5*(f1+f2)?Should I change the dimensions of the two rectangular cavities?And are the dimensions of the six rectangular cavities not the same?

Thanks in advance!