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-...
The short answer, if you're familiar with AviSynth
You don't need to use LoadPlugin(), if you copy the four files in the plugin:
to the plugins directory of your AviSynth installation (see below for its location).
That's all. Then you directly use it in your AviSynth script calling its function(s), mostly the ...