The principal objective of the radar sounder MARSIS experiment is to look for ice and water in the Martian subsurface. One particular focus of investigations, since 2005, has been the search for basal liquid water in the south polar layered deposits (SPLD). Anomalously strong basal echoes detected from four distinct areas at the base of the deposits at Ultimi Scopuli have been interpreted to indicate the presence of bodies of liquid water in this location, beneath a 1.5 km thick cover of ice and dust. Other explanations for the bright basal reflections have been proposed, however, including the possibility of constructive interference in layered media. Here, we test this mechanism through simulations of MARSIS radar signals propagating in models of CO2-H2O ice sequences. We then compare the results to real MARSIS data acquired over Ultimi Scopuli, finding that no CO2-H2O ice model sequence reproduces the set of real data. The results of our work have implications in relation to the global CO2 inventory of Mars.