Police believe Judith Ducker, 64, was injured during a raid at Wellgate News, Rotherham, on 1 September. She died in hospital on Thursday.
Paul Blueitt, 36, from Rotherham, has been charged with attempted murder and robbery.
Police are trying to locate the man and woman captured on CCTV at the time of the raid.
Officers said the couple were captured on film between 11:00 BST and 11:25 BST and they may have entered the shop and been the last people to have seen Mrs Ducker.
South Yorkshire Police said the witnesses were "crucial" to the investigation.
The force added it was waiting for further tests after a post-mortem examination failed to establish a cause of death for Mrs Ducker.
She is believed to have had underlying health conditions.
Mr Blueitt was remanded in custody at Sheffield Crown Court on 3 October.
Police investigating the death of a woman after a robbery are seeking two potentially "crucial" witnesses.