I have not researched it, but I think the confusion is stemming from the fact that the two are being tried together, but they are not being represented together (in fact, the two accused are basically antagonists here).
It's certainly true that in a regular trial, once the Crown concludes its presentation of evidence, if the defense then declines to call witnesses, they don't get to come back a few days later and say, Gee, we changed our mind. If the defense doesn't call evidence, the trial then moves to the next phase.
But in this trial, there are two accused.One argument that I've heard is that, if the co-accused presented new evidence, the other co-accused has a right to respond to that evidence with evidence (or witnesses) of his own.Now, we haven't heard whether DM's team in fact wants to do this, it's all speculation. However, I think it quite possible that IF DM's team wants to now call evidence to refute MS's testimony, they can make an application to do so, and the judge will rule on it in a voir dire.
I'm sure we'll find out next week what the story is. As I said elsewhere, I doubt DM's lawyers want him on the stand.
I haven't researched this particular issue, but I have followed a couple of high-profile trials in detail (Truscott, Morin, the Charles Smith victims) in great detail, reading all the trial transcripts etc., and remember some very anomalous requests being made to the court, (which were then discussed in voir dire and ruled upon by the presiding judge) so I don't see why, if there were a compelling legal reason to do it, a similar irregularity couldn't be allowed in this case.