With two fans you have the weight penalty if you are going for speed
planes such as the Electra single edf unit is a very slick low drag airframe.
when you add twin tail fins and wider airframes then you have the drag penalty.
The F-16 is a well designed full scale single engine fighter. Planes based on that same plan form do
very well as models. If your favorite plane happens to be a twin you can still build it
and install a single fan, you save weight and still have good performance.
The yellow Aircraft small F-18 flys great with a single 120mm fan. Some designs like the
Mig 29 will not allow a single and still look scale but it can be done.

P. Richards aka SwatTeam