  Linear stability theory (LST) is often used to model the large-scale flow
structures in the turbulent mixing region and near pressure field of high-speed
jets. For perfectly-expanded single round jets, these models predict the
dominance of $m=0$ and $m = 1$ helical modes for the lower frequency range, in
agreement with empirical data. When LST is applied to twin-jet systems, four
solution families appear following the odd/even behaviour of the pressure field
about the symmetry planes. The interaction between the unsteady pressure fields
of the two jets also results in their coupling. The individual modes of the
different solution families no longer correspond to helical motions, but to
flapping oscillations of the jet plumes. In the limit of large jet separations,
when the jet coupling vanishes, the eigenvalues corresponding to the $m=1$ mode
in each family are identical, and a linear combination of them recovers the
helical motion. Conversely, as the jet separation decreases, the eigenvalues
for the $m=1$ modes of each family diverge, thus favouring a particular
flapping oscillation over the others and preventing the appearance of helical
motions. The dominant mode of oscillation for a given jet Mach number $M_j$ and
temperature ratio $T_R$ depends on the Strouhal number $St$ and jet separation
$s$. Increasing both $M_j$ and $T_R$ independently is found to augment the jet
coupling and modify the $(St,s)$ map of the preferred oscillation mode. Present
results predict the preference of two modes when the jet interaction is
relevant, namely varicose and especially sinuous flapping oscillations on the
nozzles plane.
