Chief Engineer of the Beech 18 design, Ted Wells, recalling the decision to use twin vertical stabilizers, admitted that neither he nor his small engineering staff had any experience in analyzing the stresses imposed on the rear fuselage of a single tail, twin-engine, all-metal, semi-monocoque structure when it suffered an 'engine-out' situation. They knew that great torsional loads were placed on the rear fuselage in that situation but were unsure how to accurately do the stress analysis. They were confident analyzing stresses present in single-engine, single-tail aircraft. Engineers in the other companies in that Bureau of Air Commerce competition, Lockheed and Barkley-Grow, came to the same conclusions and were using twin tails. Large aircraft companies (Martin, Curtiss, Boeing & Douglas) with large engineering teams, were designing military single tail multi-engine aircraft.
The above is from my book, "Beech 18: A Civil & Military History" page 21 which noted its source as a videocassette interview with Wells in the History of Aviation Collection, Wichita State University, Spl Collections, #20-602 #9.
Regards, Bob Parmerter