"it" doesn't split anything to cores. the os does. and windows 7-10 have had support for multiple cores. people who didn't know this have posted over and over, over the years, saying that WoW is only optimized for 1 core, or in your case, only optimized for one core, and uses the one to split threads. Wow doesn't do this, this isn't how any of this works.
http://wowwiki.wikia.com/wiki/CVar_processAffinityMask
just educate yourself please
there is an internal setting with windows 7 that you had to change, i couldn't tell you what it was because I don't remember, that allowed for the use of multiple cores, and I did that with my 8 cores
windows 10 does it for me
just ugh why do people think they know shit ffs