We plan large Nancy Grace Roman Space Telescope and Vera C. Rubin Observatory surveys that contain several nearby Milky Way -like galaxies (at distances where the stars themselves are not resolved, i.e., beyond 20 Mpc). These observations will allow us to understand the frequency and properties of tidal substructure around more distant external galaxies based on integrated (i.e., unresolved) diffuse light. These observations therefore will complement and expand on resolved stellar population studies of very nearby galaxies. We intend to compare the observed structures to the predictions of cosmological models of galactic halo formation (e.g., the number and properties of streams around Milky Way-like galaxies). These comparisons will help us to infer the properties of stream progenitors (masses, dynamics, metallicities, stellar populations).