The twin tail plane was the one with the fins too small. This is when I learned that 2 fins fairly close together are like biplane wings without a wide spacing, they aren't as effective. I calculated the size of the fins & rudders and split it in two.

The plane took off OK, but slid sideways in turns rather than tracking around. When doing a stall turn, you could do a 360, turn, although you could also get into a flat spin. In level flight at half power, you could do a tight, flat spin/turn. It would swap ends with full rudder.

We added 1 1/2" to the top of the fins and an inch to the leading edge and also added end plates to the fins. This cured all the problems.