Douglas Frederick Ayton first abused the girls when they were aged 11.
Ayton, 62, forced them into sexual acts in his home in Londonderry, his holiday home in County Donegal in the Republic of Ireland, and in his car.
One of the attempted rapes took place as his wife was in hospital giving birth to one of their daughters, the court heard.
The abuse began after Ayton, of Donnybrewer Road, befriended the girls' families, allowing him to spend extended time alone with them.
He carried out most of the assaults while his wife was at work in a pub, but some of it happened as she and their children were present in the family home.
The abuse started in late 1981 and continued until early 1987, but only came to light in 2012 when one of the victims contacted police.
A second woman then came forward. Both are entitled to lifelong anonymity.
The judge at Antrim Crown Court said the abuse had been "systemic and sustained... robbing the girls of any normality throughout their teenage years".
His behaviour was "exploitative, manipulative and abhorrent", she added.
In a statement read to the court, one of the women said she was so "brainwashed" by Ayton that she would have "done anything for him" when in reality he "repulsed" her.
Ayton had initially pleaded guilty to only half of the charges, but changed his plea on a further 18 in May.
He had told police he had not abused the girls, but it had been a "two-way thing", and claimed he and the girls were "good together".
Ayton was given two consecutive seven-year sentences for the 33 charges of attempted rape and sexual assault.
He was given a further six months for the abuse of one of the girls' schoolmates.
He will also have to sign the sex offenders register.
A man who sexually abused two girls in a five-year period in the 1980s has been jailed for 14-and-a-half years.