DFT electronic calculations are performed for the double proton transfer in bicyclic oxalamidines 2,2 ' -bis-(3,4,5,6-tetrahydro-1,3-diazine) (OA6) and 2,2 ' -bis-(4,5,6,7-tetrahydro-1,3-diazepine) (OA7). Bidimensional potential energy surfaces retaining the characteristics of the stationary points located in the whole potential are built to calculate the vibrational levels through the discrete variable representation (DVR) method. Zero-point tunneling splittings are similar for both systems but for a given energy there are much less tunneling doublets available in OA6, this being the key factor responsible for the different temperature dependence of the rate for the two systems.