  The identification and isolation of two WIMP dark matter (DM) components at
colliders is of wide interest on the one hand but extremely challenging on the
other, especially when the dominant signal of both DM components is of the
mono-X type ($X=\gamma, Z, H$). After emphasizing that an $e^+e^-$ collider is
more suitable for this goal, we first identify the theoretical principles that
govern the occurrence of two peaks in missing energy (ME) distribution, in a
double-DM scenario. We then identify a variable that rather spectacularly
elicits the double-peaking behaviour, namely, the plot of bin-wise statistical
significance ($S/\sqrt{B}$) against ME. Using Gaussian fits of the histograms,
we apply a set of criteria developed by us, to illustrate the above points
numerically for suitable benchmarks.
