In this work, a novel combined diagnostic capable of measuring multiscale density fluctuations that extend from magnetohydrodynamic (MHD) to the lower range of electron temperature gradient turbulence has been designed, installed, and operated at DIII-D. The combined diagnostic was constructed by adding a heterodyne interferometer to the pre-existing phase contrast imaging (PCI) system, both of which measure line-integrated electron density fluctuations. The port-space footprint is minimized via use of a single 10.6 μm probe beam. With temporal bandwidths in excess of 1 MHz, the PCI measures high-k (1.5 cm-1 < |k R | ≤ 25 cm-1) fluctuations with sensitivity , while the interferometer simultaneously measures low-k (|k R | < 5 cm-1) fluctuations with sensitivity . The intentional mid-k overlap has been empirically verified with sound-wave calibrations and allows quantitative investigation of multiscale effects that are predicted to be significant in the reactor-relevant T e ∼ T i regime. Furthermore, via correlation with the primary DIII-D interferometer, the toroidal mode numbers of core-localized MHD can be measured.