I think my theory solves a few problems. Both Darth Plagueis and Darth Sidious "created" Anakin, sort of, but really it was the Force. Darth Plagueis attempted to manipulate midichlorians to create the Chosen One with the help of his apprentice, Sidious, who realized that this would mean his replacement as apprentice.
So having taught Sidious all that he knew, Plagueis was really dispensable, and Sidious killed him and tried to do create life himself. Only Plagueis didn't teach Sidious everything he knew, that is, he taught him all the methods to create life, but didn't tell him that a powerful Sith could control his midichlorian essence after death and be reincarnated into a new body.
So, having given his apprentice all of the tools to create a powerful midichlorian life form, he allowed himself to be killed by Sidious to accomplish his plan. (c'mon, how else is a Sith Lord going to get killed in his sleep by his apprentice).
Sidious, believing that he had rid himself of his master, and obviously not loyal to his apprentices (Maul, Dooku), set out to create for himself this powerful chosen one as his new apprentice.
But in order to ensure that such a force sensitive child wouldn't be discovered by the Jedi, he had to go to the outer rim and get a person to experiment on, naturally a slave, Shmi Skywalker from Tatooine. He used the power of the force to impregnate her with high density midichlorian essences (Plagueis).
But neither he, nor Plagueis realized that their Sith wills weren't stronger than the Force itself, so instead of all of the dark side midichlorian Plagueis Sith essence being reincarnated into Anakin, the Force provided half light side midichlorians, thus bringing forth a Force Balanced creation in Anakin.
None of this mind you would have been possible if the Force had not willed it, which is why Sidious couldn't repeat the experiments later.