Ian Huntley and Maxine Carr were both liars, the prosecution has said in the final stage of the Soham murder trial.
The prosecution says Mr Huntley killed the girls
Richard Latham QC, summing up the prosecution's case, said Mr Huntley was a "capable and convincing liar".
The former school caretaker denies murdering 10-year-olds Holly Wells and Jessica Chapman last year, but admits they died in his house.
He admits conspiring to pervert the course of justice.
Ms Carr, Mr Huntley's ex-girlfriend, denies conspiring to pervert the course of justice and two counts of assisting an offender.
The jury will begin considering its verdicts on Friday, Judge Moses said after Mr Latham finished his closing speech.
Mr Latham told the Old Bailey both girls "had to die" in Mr Huntley's "own selfish self-interest" after his sexual motive for taking them into his home went "plainly wrong".
"Each were potential witnesses - he was quite merciless," Mr Latham told the jury in his closing statement on Wednesday.
He said Mr Huntley had "nerves of steel" to carry out a series of media interviews knowing the girls were dead and to actively seek out Holly's father Kevin to speak to.
The defendant also conducted "a series of ruthless acts" by dumping the bodies of the girls in a remote ditch and cutting off their clothes to remove potential evidence that would link him, Mr Latham said.
The caretaker's apparent helpfulness after the girls disappeared was "12 days of cynical deception".
These were the actions of "a thinker, a planner, a man who is acting calmly and rationally," he said.
Mr Latham said Ms Carr, during the media interviews, was cynically relaxed and chatty in her manner "as she lies through her teeth about the important thing - that she was present on the Sunday, which she was not".
Ms Carr withheld information about the house she shared with Mr Huntley and her movements, Mr Latham said.
"She lied to every single person to whom she spoke about this... not just up to her arrest, but right the way through to the end of the interviews."
During his evidence Mr Huntley claimed Holly died accidentally after falling into his bath last August when he was helping her with a nosebleed.
However, he admitted he had killed Jessica by putting his hand over her mouth to stop her screaming and that he had told lies to police to cover his tracks.
He also admitted dumping the girls' bodies in a ditch and trying to burn them.
Mr Latham said that he did not expect the jury to be precise in identifying in which room and how both girls died - "all you have to be sure about...is that he murdered each girl".
He told the jury that defence admissions are likely to be true, but "if you conclude he has lied to you...we say you can safely conclude he murdered both of those girls."
Pinned to ground
Mr Latham said it was not possible for Holly to have drowned in six to eight inches of bath water with two other people within arm's reach.
He also said Mr Huntley's version of Jessica's death - where she collapsed after he used one of his hands to stop her screaming - was unlikely.
"We suggest she was pinned to the ground, or to the wall or she was gripped with a vice-like hold with the other - important - hand for a long time"
The jury will be provided "unusually" with a laptop for their deliberations.
The laptop will contain all images and plans relevant to the case, and all the media interviews so they can be watched, Mr Latham said.
The case continues.