I'm not sure what a filter function refers to. To measure something using dB you have to compare two things. If they are the same then 0 dB is the way to describe that relationship.
When I think of a time-domain "filter function" I think of the impulse response of the filter. If that's what you mean, then you are describing a filter that, when hit with an impulse, rings at two perfect frequencies with no transient. That doesn't make sense.
Please describe what YOU mean by "filter function" more explicitly -- it might just be a matter of different terminology.