in the ADF4159+ADF5901,who decide the VCO’s frequency ?

We use ADF4159 and ADF5901 to produce RF signal.In ADF4159 datasheet,we can set register to set vco’s frequency,and In ADF5901 datasheet,we can set register to set vco’s frequency.the question is:if in the ADF4159+ADF5901,we set different VCO’s frequency, who decide the VCO’s frequency ? in the ADF4159+ADF5901application,we comprehend the workflow of the ADF4159 and ADF5901 :first set vco’s frequecny of the ADF5901,for example,24GHZ,secondly,we repeatly set different VCO’s frequency of ADF4159,the ADF5901 can output signal at corresponding frequency.please check my comprehension is right or not?