Raelene Polymiadis appeared in Adelaide Magistrates Court on Wednesday.
Prosecutors alleged the 62-year-old had administered her own insulin to Brenda Anderson and Lynton Anderson, both aged 94.
Mrs Anderson died in hospital in March last year.
Her husband died after being found unconscious by a carer at his home in May this year.
Polymiadis was charged with the murder of Mrs Anderson on March 16, 2022, and with the murder of Mr Anderson on May 1.
Last week, Detective Inspector Mark McEachern said concerns over Mrs Anderson's death were first raised during a coronial investigation, which unearthed alarming toxicology results.
A similarly concerning toxicology report was returned after Mr Anderson's death.
"The toxicology in both people was quite similar in so far as raising our suspicion that the medication that was in there should not have been in there," Insp McEachern said.
Detectives had no concern with the level of care Mrs Anderson received while in hospital.
They had also ruled out carers being involved in Mr Anderson's death.
Police previously asked to speak to anyone who had contact with the couple, particularly those with knowledge of any prescription or non-prescription medication they were taking.
They said they had been working with the couple's broader family, who had been shocked by the forensic findings.
Polymiadis had a bail application rejected and was ordered to return to court later this year.
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