Eight-week-old Abygail and Madysen Fitterer, who are joined at the abdomen, had tissue expanders inserted during a four-hour procedure on Tuesday, the Mayo Clinic announced Wednesday.
After two weeks, the expanders will slowly be filled with saline solution to give each girl enough skin after separation to close their incisions.
The separation will likely take place before the end of the year, according to the hospital's statement.
Mayo teams separated another set of conjoined twins from North Dakota — Abbigail and Isabelle Carlsen — on May 12. Thirty staff members participated in the Carlsens' daylong surgery led by Dr. Christopher Moir, who will also lead the Fitterer girls' surgery.
Parents Stacy and Suzy Fitterer of Bismarck, North Dakota, said in the news release that they were encouraged by the Carlsen case.
They said they will not talk to reporters and will instead focus their energy on their daughters, born Aug. 8.
"We're proud of our little girls," Suzy Fitterer said in the release. "They're strong, and they've made it through each step of this process so far with flying colors."
Moir called Tuesday's tissue-expansion surgery "a good dry run" for the separation.
"Abygail and Madysen came through yesterday's operation and bounced back really well. That was an important step," he said.