The cross sections for the production of tt-bar bb-bar and tt-bar jj events and their ratio sigma[tt-bar bb-bar]/sigma[tt-bar jj] are measured using data corresponding to an integrated luminosity of 2.3 inverse femtobarns collected in pp collisions at sqrt(s) = 13 TeV with the CMS detector at the LHC. Events with two leptons (e or mu) and at least four reconstructed jets, including at least two identified as b quark jets, in the final state are selected. In the full phase space, the measured ratio is 0.022 +/- 0.003 (stat) +/- 0.006 (syst), the cross section sigma[tt-bar bb-bar] is 4.0 +/- 0.6 (stat) +/- 1.3 (syst) pb and sigma[tt-bar jj] is 184 +/- 6 (stat) +/- 33 (syst) pb. The measurements are compared with the standard model expectations obtained from a POWHEG simulation at next-to-leading-order interfaced with PYTHIA.