When Voldemort tried to kill Harry in the first book, a fragment of his soul split off and continued to live in Harry. Harry had his own whole soul and the part of Voldemort's soul in him from then on.
Due to him having more than one soul in him, he could not be killed by Voldemort in the Forbidden Forest in the seventh book. Voldemort's Killing Curse merely destroyed the fragment of his soul inside Harry, but Harry's own soul stayed intact.
Harry basically had two lives and Voldemort's Killing Curse destroyed one of them. Couldn't any dark wizard force others to put a bit of their soul into his body to have multiple lives?
For example, dark wizard A forces servant B to perform the Horcrux making procedure. B doesn't put the split off soul fragment into an object though but instead into A. A now has his soul and additionally a part of B's soul -> Two lives. If A was hit with the Killing Curse, just one soul would be destroyed and A could live on with one life less.
Does this make sense or have I missed something?