Differences between the spectroscopic ﹩{M}_{K{\rm{s}}}﹩ estimates derived from mock composite binary spectra vs. those for the primaries of the composites. Red symbols show OB+OB binary systems and the gray symbols are binary systems composed of an OB star and a companion with another (AFGK) spectral type. In most cases, the composite spectra cause the inferred spectroscopic ﹩{M}_{K{\rm{s}}}﹩ to be slightly fainter than the primary, further facilitating the identification of binaries through the difference between geometric ﹩{M}_{K{\rm{s}}}﹩ and spectroscopic ﹩{M}_{K{\rm{s}}}﹩. For example, the geometric ﹩{M}_{K{\rm{s}}}﹩ for equal-mass binaries are 0.75 mag brighter than their primaries (dashed line in blue), due to the contribution from the secondary. We note that for some binaries composed of a late-B type primary with ﹩{M}_{K{\rm{s}}}\gtrsim 0.5﹩ mag and an AFGK-type secondary, the spectroscopic ﹩{M}_{K{\rm{s}}}﹩ could be brighter than that of the primary. As such, the binary identification in this regime is less efficient (see text for details).