We test whether the Tinker & Chen model of Mg ii absorption due to the gaseous halo around a galaxy can reproduce absorption in quasar pairs (both lensed and physical) and lensed triples and quads from the literature. These quasars exhibit absorption from a total of 38 Mg ii systems spanning z= 0.043–2.066 with mean redshift 〈z〉= 1.099 and weighted mean rest-frame equivalent width of 0.87 Å. Using the Tinker & Chen model to generate simulated sight-lines, we marginalize the unknown parameters of the absorbing galaxies: dark matter halo mass, impact parameter and azimuthal angle on the sky. We determine the ability of the model to statistically reproduce the observed variation in Mg ii absorption strength between paired sight-lines for different values of the gas covering fraction fc and the characteristic length scale ℓc, within which the variation in absorption equivalent widths between sight-lines exponentially decreases. We find a best-fitting fc= 0.60 ± 0.15 per cent and (1σ confidence limits), with smaller fc allowed at larger ℓc. At 99.7 per cent confidence, we are able to rule out fc > 0.87 for all values of ℓc and the region where kpc and fc < 0.3.