The similarity hypothesis is that the seasonal pattern of deviation from mean CO2 mass in each cap is of the same shape, with an intercap phase difference of half a Martian year. We thereby decompose the annual pressure variation into separate components of CO2 exchange with each cap. Sublimation at one cap and condensation at the other tend to occur simultaneously. The annual fluctuation of CO2 mass in the two caps thus greatly exceeds previous estimates based on treating each cap in isolation. Inferred behavior is generally consistent with optical data on cap recession. The seasonal pattern of meridional mass flux of CO2 is estimated.