The case also involves the suspicious deaths of both of their former spouses.
Madison County Prosecutor Rob Wood argued that having two trials could result in the second trial's jury being prejudiced by the first trial jury's verdict. Attorneys for the Daybells could ask in the future that the trials be separated if they think their clients aren't being treated fairly.
The couple each pleaded not guilty.
Prosecutors said the couple, who reportedly hold apocalyptic beliefs, conspired to hide or destroy the bodies of 7-year-old Joshua “JJ” Vallow and 17-year-old Tylee Ryan.
Rexburg Police Detective Ray Hermosillo said in August that “JJ” Vallow’s body was bound with duct tape and buried in a black plastic bag.
The remains of daughter Tylee were found burned a short distance away.
In June 2019, Lori Daybell's estranged husband, Charles Vallow, was shot and killed in Arizona by her brother, Alex Cox. Vallow was seeking a divorce because Daybell believed she was a godlike figure who would bring on the apocalypse.
After the death of Vallow, Daybell and the children moved to Idaho, where she eventually married Chad Daybell. Chad Daybell's previous wife died in October 2019 of natural causes, according to her obituary. He also held apocalyptic beliefs, telling friends that he could see "beyond the veil."
The Daybells reportedly believe dark spirits or "zombies" can possess people, and that the only way to get rid of these dark spirits is to kill the person. A friend of Lori Daybell's said she thought the two children were possessed.
Investigators said the couple lied about the whereabouts of the children before quietly leaving Idaho for Hawaii last year. They were arrested earlier this year.
The Associated Press contributed to this report.