Group velocity dispersion for coupled waveguide system
You generally wouldn't try to specify group velocity dispersion for such a system. Instead, you'd simply measure (or estimate from simulation) S-parameters for the 4-port network formed by a certain length of coupled waveguides. 

From there you could determine a group delay for signals propagating from one port to any of the other 3 (or reflecting back to the input port for that mater). But you shouldn't expect this delay to grow linearly as the length of the coupled section is increased, so it probably doesn't make sense to talk about group velocity of the structure.