Instead of using ferret for that, you may use the ‘cdo’ (NetCDF operators) with the merge and copy or cat options:
cdo merge u1_wind.nc v1_wind.nc uv1_wind.nc
cdo merge u2_wind.nc v1_wind.nc uv2_wind.nc
cdo copy uv1_wind.nc uv2_wind.nc uv_wind.nc
You can make a whole loop to cycle thru all files. CDO has an extensive documentation and has compatibility support for Ferret and GrADS. See CDO’s manual pages 25 and 27 (chapters 2.2.1 and 2.2.3). Sorry if that was not the solution you were looking for, but it is very simple once you have CDO handy. Ubuntu at least can install cdo via an apt-get.