"Combined Angiotensin Inhibition for the Treatment of Diabetic Nephropathy"  "The study was stopped because of increased risks of hyperkalemia and acute kidney injury with combination therapy."
NEJM
http://www.nejm.org/doi/full/10.1056/NEJMoa1303154?query=featured_home 
"CONCLUSIONS

Combination therapy with an ACE inhibitor and an ARB was associated with an increased risk of adverse events among patients with diabetic nephropathy."