Prosecutors as a general rule cannot “promise less time in prison”. Sentencing is not up to them. What they can do in some cases is to make lesser charges: manslaughter, for example, instead of criminal murder, or negligent homicide instead of manslaughter. The sentencing guidelines differ for the crimes, but there can be overlap, too, and a judge who hears the case can, after the jury brings in a “guilty” verdict (and assuming that happens, if there is a trial) make a maximum sentence based upon what he hears.
Of course, if a plea deal is reached, then an allocution as to the facts and events of the crime is generally part of the deal: the guilty person has to say out loud – to confess – “what he did”, and perjury is still a chargeable offense separate from the crime that is being confessed.
Video tapes can’t always show conclusively that two criminals ‘did something equally’. Mens rea – “guilty mind” – is part of every actual crime: did the person know that what he was doing was a crime? Did he act freely and without coercion of some kind? Was he sober, sane and in control of his thoughts and actions? Videotape doesn’t show that, and most camera evidence is inconclusive even in the best cases.
Also in general when a prosecutor makes a deal for one criminal to turn state’s evidence on another, the deal goes to whoever makes the first admission of guilt, and implicates the other one. The prosecutor isn’t there to finely balance the scales of justice. He’s there to move meat through the system and make convictions without committing gross injustice (at least provable corruption) himself.
This whole scenario is the basis of the Prisoner’s Dilemma, which is a relevant part of Game Theory in these cases.