Actually the two fins and rudders have about 1/3 more area than the single rudder.

Single fin: 50 squar inches
Single rudder: 29 square inches.

Double fin: 85 square inches
Double rudder: 44 square inches

See Post 71 (I think)

I had been reading STM threads and many people complained about not having enough rudder authority. And had how many had extended the size of both the fin and rudder. The double rudder was my way of increasing rudder authority without increasing profile. Plus.. I was tinkering with the idea of making an STM twin, but I wanted to get my feet wet with a twin-tail first.