[en] We predict that, due to a many-body coupling between collective and single-particle excitations, there is an experimentally observable resonant line splitting in the intersubband collective charge-density excitation spectra in wide semiconductor quantum wells at low electron densities. This many-body coupling causes even the long-wavelength intersubband plasmon mode to be Landau damped at a critical density. Above the critical density, the intersubband plasmon mode lies lower in energy than the single-particle excitation