As it is impractical to enumerate all possible pivot sentences, the two NMT models are trained separately in practice: Due to the exponential search space of pivot sentences, the decoding process of translating an unseen source sentence x has to be divided into two steps: The above two-step decoding process potentially suffers from the error propagation problem (Zhu et al., 2013): the translation errors made in the first step (i.e., source-to-pivot translation) will affect the second step (i.e., pivot-to-target translation).
it is necessary to explore methods to directly model source-to-target translation without parallel corpora available.