What does it mean by " concentrations are (value) in relation to each other"?
The cation Ag+(aq) and Hg2^2+ are in an aqueous solution. Their
  concentrations are 0.2 moldm^-3 in relation to each other. Find which
  cation precipitates first as its iodide when a solution of NaI is
  added gradually to the above solution


It is a poorly worded question. The author means that the ratio of Ag conc. to Hg conc. is 0.2. It is implied that the concentration of silver is lower. Which ion will ppt out first, if we add NaI?

The author should have written that the relative ratio of Ag:Hg concentration is 0.2.