Two-centre interference in the recombination step of molecular high harmonic generation (HHG) has been probed in CO2 and O2. We report the order dependence of characteristic enhancements or suppressions of high harmonic production in aligned samples of both molecules. In CO2, a robust destructive interference was seen consistent with the known separation of the oxygen atoms that are active in HHG. In O2, a harmonic enhancement was found indicating constructive interference. A good agreement was found with a simple two-centre interference model that includes the angular distribution function of the sample. The effective momentum of the electron wave was determined from the spectral position of these interferences. Ellipticity-dependent studies in CO2 clearly show how the destructive interference can be 'switched off' by increasing the degree of ellipticity and thus shifting the effective resonance condition.