Imendansas ne rezonansas :) I suggest an absolute minimum of about 1.5 octaves between the driver resonance and crossover frequency. A tweeter with a 900Hz resonance should therefore be crossed over at a minimum of 2,500Hz, but preferably higher. If you use the minimum possible frequency separation, there will be a small peak at tweeter resonance - this is a combination of the tweeter's resonance itself, and the fact that the crossover cannot maintain the correct rolloff if the load impedance changes.