  We present families of beating dark-dark solitons in spin-orbit (SO) coupled
Bose-Einstein condensates. These families consist of solitons residing
simultaneously in the two bands of the energy spectrum. The soliton components
are characterized by two different spatial and temporal scales, which are
identified by a multiscale expansion method. The solitons are "beating" ones,
as they perform density oscillations with a characteristic frequency, relevant
to Zitterbewegung (ZB). We find that spin oscillations may occur, depending on
the parity of each soliton branch, which consequently lead to ZB oscillations
of the beating dark solitons. Analytical results are corroborated by numerical
simulations, illustrating the robustness of the solitons.
