There is no problem with having installed both 32- and 64-bit version of AviSynth on your 64-bit machine, if you correctly placed 32-bit plugins vs. 64-bit ones to their appropriate places.
AviSynth is a frame server, it means, you never call it directly - other program call it back. It means that for example
- 64-bit Media Player Classic will call the 64-bit version of AviSynth,
- 32-bit Media Player Classic will call the 32-bit version of AviSynth.
- 64-bit FFmpeg will call the 64-bit version of AviSynth,
- 32-bit FFmpeg will call the 32-bit version of AviSynth.
I guess that your problem lies in your FFmpeg installation — you have probably 64-bit, and it means that in the process of converting your .avs script to some media file the FFmpeg calls 64-bit version of AviSynth, and you need the 32-bit (because of your 32-bit filter).
So my advice is, install both 32- and 64-bit versions of FFmpeg (into different directories) and use the appropriate one for every particular case.