They were sentenced to 10-and-a-half and 11-and-a half years respectively at Middlesex Crown Court.
The couple, from Maida Vale, London, admitted three counts of child cruelty.
They had denied charges of causing grievous bodily harm with intent between 1 February and 18 March last year, but were convicted at the end of a trial last December.
The little girl had boiling water poured over her hands, had huge clumps of hair ripped from her head, was kicked so hard in the groin she suffered internal injuries and was locked naked in the toilet each night.
She was also forced to eat her own faeces.
Det Sgt Anthony Smith, from the Central Child Abuse Team, said: "This is without doubt the worst case of child abuse I have personally had to deal with.
"When I met the young girl she was in a terrible condition.
"But I'm now happy to report that she's improved dramatically with the care that she's now receiving."
'Would have died'
Jurors heard the youngster was in almost constant agony until her grandmother began to suspect what was going on and called in social services.
The little girl had suffered such "horrific injuries" that she would almost certainly have died without treatment, the court heard.
She was in such pain that she had to be examined by doctors under general anaesthetic.
The court heard how the little girl, who cannot be named for legal reasons, was born prematurely and afflicted with cerebral palsy as a result.
Shortly after her second birthday she was taken into care following concerns about domestic violence.
She was due to be fostered to a family but a report from Westminster Council in January last year recommended the girl be placed back into the care of Duncan and Harte.
Although it is understood up to 20 visits were made to the family home by social services and other care professionals, no abuse was suspected.
But the court heard that, within a month of being placed back with her family, the girl was being abused by her carers.
Passing sentence, Judge Paul Worsley told the pair: "The physical scars may well heal in time, but I doubt the mental scars ever will.
"No tear of remorse has been shown by either of you for what you did."
The girl's injuries have led to urgent calls for a full independent inquiry into Westminster Council's handling of the situation.
Councillor Paul Dimoldenberg, Leader of Westminster's opposition Labour Group, said: "We believe there needs to be an inquiry into the circumstances of how this baby girl was allowed to go back to her parents."
"There appears on this occasion to have been no shortage of experts involved but they appear to have got it very badly wrong."