If you use Groucho2004 "Universal Avisynth Installer" :-
https://forum.doom9.org/showthread.php?t=172124
You can install both x86 and x64 Avs+, and with the appropriate mvtools2.dll in each of the autoload plugin directories, then there would be no problem (ie you dont need to load it).
Dont know if Grouchy will still be around when AVS+ x128 will be unveiled [might have moved on from D9, or be "as dead as a doornail", would be a nuisance either way.].