There are more than two civvy TFO's currently employed by NPAS. And as each region joins, more will be TUPE'd across.
However I suspect NPAS actually prefer to have Police Officers as TFO's as they have far more control over them, for instance it's not in the civvy contract that they will cover at other bases when required, yet it is in the Police Officer's secondment contract that they do.
The current recruiting drive for NPAS's TFO 'succession plan' has only been made available internally to substantive Police Officers. They are not recruiting civillian TFOs (at least at the moment).