If the fuselage is not a factor as it is in the F-15, F-18, etc. then the single vertical fin will be a little more effective than two small ones of the same total area.

This is because big wings basically work better than small ones for a lot of reasons. However just add a little more area to the twin fins and you have about the same thing. I don't know what the ratio of single to twin would be though.

The main question though is how big should be vertical fins be to start with. On the F-15 project we performed wind tunnel tests using a slab flat surface and we would add or subtract area until we got the directional stability that our analysis had indicated we needed. Then we would make a normal airfoiled surface and verify that we had what we thought we had!

In a model it is a different case as it is mostly cut and try.