If your software supports virtualization of the config.wtf file, then you only want/need the one install.

If it doesn't, then you'd use symbolic links for the Cache, Data and Interface folders, meaning a single install at full size, and one copy of 10-50MB (the size of your addon folder basically) per different level of settings you'd like; probably 1x Everything on (solo boxing), 1x Half Settings (the master), 1x Low Settings (all the slaves).