Within a neuron, spike propagation can occur in a complex manner, with spikes propagating into some processes but not others. We study this phenomenon in an experimentally advantageous mechanoafferent in Aplysia, neuron B21. B21 has two processes within the CNS. One is ipsilateral to the soma and is referred to as the lateral process. The second travels into the contralateral hemiganglion and is referred to as the contralateral process. Previously we characterized spike propagation to the lateral process, which is an output region that contacts follower motor neurons. Spikes fail to actively propagate to the lateral process when B21 is peripherally activated at its resting potential. This propagation failure can be relieved if the medial regions of B21 are centrally depolarized during peripheral activation. The current study examines spike propagation to the contralateral process. We show that, unlike the lateral process, active spike propagation in the contralateral process occurs when B21 is peripherally activated at its resting membrane potential. Thus spike propagation occurs selectively, favoring the contralateral process. Interestingly, the contralateral process of one B21 is immediately adjacent to the medial region of the bilaterally symmetrical cell. The B21 neurons are electrically coupled, suggesting that spikes propagating in the contralateral process of one cell could modify propagation in the sister neuron. Consistent with this idea we demonstrate that lateral process propagation failures observed when a single B21 is peripherally activated can be relieved by central coactivation of the contralateral cell. These results imply that stimuli that coactivate the B21 neurons bilaterally are more apt to generate afferent activity that is transmitted to followers than stimuli that activate one cell.