On the money
etudiant. The to be famous Kelly Johnson did the majority of the wind tunnel work on the Lockheed 10. It initially had a single fin and proved to be inadequate, so he suggested twin fins. What drove that decision rather than just making the single fin larger I've not seen recorded. The Beech 18 in some conversions has had its twin fins removed and replaced with a single.
Edited to add a very good insight into the 1941 thinking behind twin V single tail
1941 | 1585 | Flight Archive