The optical spectrum of mode-locked lasers can exhibit multiple peaks resulting from different mechanisms such as modulation instability, dispersive waves (DWs), and coupling between continuous waves (CWs) and DWs. The latter was recently reported in a mode-locked fiber laser. Here we show that besides the coupling between single-wavelength CW and DWs, dual-wavelength CWs can also couple with DWs giving rise to quite different spectral peaks in a mode-locked fiber laser. In particular, we find that the sidebands of one CW can couple with the other CW, leading to an enhancement of the CWs.