If a couple who worked together to commit several murders be sentenced differently depending on how they both perferred to kill their victims? One only uses guns and uses methods like a professional assassin would. While their lover enjoys being up close, personal, and very messy with a chainsaw. Usually to draw attention away from the one picking off victims from a distance. The motive also differs. The assassin arranges the jobs and recieves payment while the other is a slasher-movie cannibal.
Joint Trials for co-defendants are decided by the Judge who is overseeing the trial. Joint trials usually occur if many of the facts and pieces of evidence overlap, the defendants participated in the same acts, or the charges are closely connected in time, place, and purpose.
The fact that your two characters, while work together, are differnt in their approach may make the Judge decide to have separate trials. That is, unless each defendant cannot be tried on their own since the acts committed by the two are intertwined.
If the two characters have differnt trials, then, yes, the defendants would be sentences differently because of the aggravating circumstances (how the crime was committed). If the two are co-defendants in the same trial, they would share the same sentence if the Jury believes them to be both equally culpable of the same charges.