I like to have my heads seperate and you are right about it being easier to blendshape. But you dont have to its pretty much just as easy as having the head together.
To cut the head use you split polygone tool and make a small new edge line just above a edge that is already on the neck. Once you have made the new row select the faces, then delete the faces. Once that is done go up to edit polygons find seperate and click it.
That should do it=). Hope I made sence.
Kurt
I am enough of an artist to draw freely upon my imagination, knowledge is limited, imagination encircles the world. (Albert Einstein)
https://www.artstation.com/kurtb