Combination therapy with colistin and meropenem was not superior to colistin monotherapy for the treatment of pneumonia or BSI caused by XDR A. baumannii, P. aeruginosa, or CRE in 464 patients who were enrolled in a randomized study published in NEJM evidence.
There was no difference in mortality (43% vs 37%), clinical failure (65% vs 58%), or microbiologic cure rates (65% vs 60%) between monotherapy and combination therapy groups respectively.
https://evidence.nejm.org/doi/full/10.1056/EVIDoa2200131