How to change AviSynth plugins directory?
As the docs indicate, you have to create registry keys to tell Avisynth+ where to look. They are

PluginDir+ in Software/Avisynth in HKEY_CURRENT_USER
PluginDir+ in Software/Avisynth in HKEY_LOCAL_MACHINE
PluginDir2_5 in Software/Avisynth in HKEY_CURRENT_USER
PluginDir2_5 in Software/Avisynth in HKEY_LOCAL_MACHINE


There is no separate key for 32 or 64-bit. Since 32-bit AVS will not load 64-bit plugins, I suggest setting the first key in the above list to the 64-bit plugins folder, and the 2nd to the 32-bit. The 64-bit AVS DLL when loaded will use plugins from the first folder. The 32-bit DLL will skip them and load from the 2nd folder.