  We study theoretically the dynamics of two weakly-coupled Bose-Josephson
junctions, prepared with the same particle number $N$ and Josephson excitation
number $\nu$ but with different reduced one-particle purity $\gamma$. A novel
entropy oscillation mode is predicted, in which one-particle coherence is
transferred between the Bose-Hubbard dimers with no particle or energy
transfer. We explain this purity oscillation using a semiclassical picture.
