The 52-year-old was supposed to go stand trial in January in the deaths of 8-year-old Tyrone and 7-year-old Tina Urquhart.
Hall was charged after prosecutors said DNA evidence linked him to the crime. The same evidence was used to free another man who served 22 years of a life sentence for the murders before being exonerated.
Hall was a neighbor to the children in a rooming house where their mother lived with the man originally accused of killing them.
Hall was charged with the murders while serving time in prison for three sexual assaults in the 1990's.