Frequency components are added for time domain orthogonal kernels?

Hi,
I am reading the book of multirate signal processing written by
Harris. On page 47, the windowed filter design technique is
introduced. It mentioned the frequency domain components for two
orthogonal time domain kernels are added. It is a right conclusion for
any orthogonal time domain kernels? I cannot find the derivation for
that conclusion. Can you explain it further to me? Thanks.