You can use stickboy's JDL_UnFoldFields function to get the fields separate but displayed on the same frame. Then use JDL_FoldFields to get them back as they were.
JDL_UnFoldFieldsVertical(flip=true)
DeGrainMedian()
JDL_FoldFieldsVertical(flip=true)
Download this :
http://www.avisynth.org/stickboy/jdl-interlace.avsi and save it to your Avisynth 2.5 plugins folder. By default it is C:\Program Files\Avisynth 2.5\plugins . That way it will be auto-loaded when needed. Then try that script, if you don't know what it does, comment out (by placing a # in front of the line you comment out) line by line and you'll see what happens. No need for separating fields, weaving etc.