it cannot be forgotten that both DM and MS could have spared them a lot of the speculation had they each agreed to plead guilty
It will come out after the trial is over if my hypothesis is correct, but I would be very surprised if either or both defense counsel did not try to negotiate with the Crown to have the accused (one or both) plead guilty to second-degree murder (or manslaughter - possible, but less likely given the circumstances).
The Crown would have rejected this because they felt sure they had sufficient evidence to prosecute for first-degree murder. Thus one or the other accused may be hoping that the jury will decide that he/they are guilty of a lesser offense.
The same thing happened in the Bernardo case, but it wasn't made public till after the jury's verdict. In his case, he agreed to plead guilty to second degree murder and spare the families the ordeal of the videotapes. He was certain to be locked up indefinitely anyway because of all his rape charges (which led to his being designated a Dangerous Offender with virtually no chance of release ever).
IIRC Bernardo insisted on going to trial because he maintained that although he had done all the assaults, rapes etc. he had not actually killed the two girls (in that I agree with him -- evidence points to Homolka, but it was too late to deal with that). Rosen did a good job challenging Homolka's self-serving testimony, but in the end the tapes spoke for themselves. Even if the jury disbelieved Homolka, there was ample evidence of the assaults and because the deaths occurred as a consequence of kidnapping and sexual assault (and forcible confinement), PB was guilty of first degree murder whether or not he actually strangled the girls.
Hard to know what the motives and expectations of the current accused might be, and I wouldn't want to guess. But I do think their attorneys recommended a guilty plea and made such an offer, one that was rejected; I consider this highly likely because of the many precedents and specifically the Bernardo case.