Both girls currently face adult charges. They allegedly stabbed a friend, also 12, to appease and please "Slender Man," an online horror character.
All three were classmates in Waukesha, a city 18 miles west of Milwaukee.
Investigators said the stabbing occurred after a sleepover to celebrate the birthday of one of the suspects. The suspects allegedly lured their friend into a wooded area and then launched a deadly attack, stabbing her 19 times.
The victim was able to crawl out of the woods where she was found by a cyclist.
Lawyers for the two defendants are believed to be planning to try to have the case returned to juvenile court.