I'd hypothesize that using harmalas with MDMA would significantly increase the neurotoxicity since MDMA basically breaks open the SERT pathway and allows other neurotransmitters to be taken up and oxidize inside the neuron. Since harmalas increase the concentration of neurotransmitters in the synapse, it stands to reason one would experience a proportional increase in neurotoxicity when combining harmala with MDMA. This would be an interesting experiment for someone with the means to test it.