Inner-valence ionized states of weakly bound systems like van der Waals clusters can efficiently decay by electron emission. The mechanism of the decay, which does not occur in the isolated monomer units constituting the clusters has recently been shown to be of intermolecular/interatomic nature. This intermolecular/interatomic Coulombic decay (ICD) mechanism prevails in many systems ranging from hydrogen-bonded molecular clusters to atomic rare gas clusters. In the present paper we extend our previous studies to weakly bound heteroclusters built up of monomer units of largely differing energetics. It is shown that, as soon as the double ionization potential of a monomer unit is lower in energy than the ionization potential of the initially created inner-valence vacancy on a neighboring monomer unit, an additional electronic decay process can take place. In contrast to the ICD mechanism, which involves an efficient energy transfer between the monomer units, this second process is essentially based on an electron transfer process. It is therefore termed electron-transfer mediated decay (ETMD). We have analyzed the mechanisms of the electronic decay processes taking place following inner-valence ionization in weakly bound heteroclusters in an exemplary study of the NeAr dimer. The involved electronic states have been calculated using ab initio Green's function techniques. The lifetime of the inner-valence Ne(2s(-1))Ar vacancy has been estimated and partitioned according to the contributions of the two decay channels based on a perturbation-theoretical description of the decay process. As a result, the lifetime of the inner-valence resonance state is estimated to be of the order of 10-100 fs, the specific value strongly depending on the internuclear separation of the monomers. The ICD process is shown to be by far the dominant decay channel at distances corresponding to bound states of the dimer. With decreasing internuclear separation the ratio of the ETMD and ICD decay widths quickly increases over several orders of magnitude.