Mixed beryllium carbon dianions BeC42- and BeC62- have been examined using ab initio methods. The dianionic systems have been investigated in detail with respect to electron autodetachment as well as with respect to fragmentation. Only linear isomers have been found to represent geometrically stable isomers, i.e., are minima on the corresponding potential energy surfaces. While a linear isomer of the BeC42- dianions lies on the verge of electronic stability, a linear isomer of the BeC62- dianions, better referred to as C2BeC42-, is the smallest free stable Be-C dianion. To estimate the lifetime of the BeC42- system we have calculated the repulsive Coulomb barrier following ab initio approaches introduced in this work and used this potential to compute the tunnel probability with the help of Wentzel-Kramer-Brioullin theory. The tunnel probability makes the lifetime of the system directly accessible, when the electron detachment energy (EDE) is known. All calculated EDEs yield lifetimes markedly longer than 5 mu s, which is the lower limit for experimental observation in a mass spectrometer.