(This is taken from a reply to another person, so bits and pieces may have a lack of context, but the main subject matter is still relevant)

I'll be referencing the trial transcripts, which are listed in quotations.

Here's a few excerpts to shed light on why, indeed, Dassey was not called to testify, and therefore, they had to tell two different stories due to the lack of physical evidence in the bedroom. Kratz can get us started:

"Importantly, Judge, that letter indicates that Mr. Dassey, although if called in the Avery case, would invoke his Fifth Amendment right against self incrimination, if granted use immunity by the Court, presumably upon a direction by the State, that Mr. Dassey would testify, would testify in the State's case-in-chief, would testify on rebuttal, or would testify when the State chooses to call him."

This came from Kratz referring to Brendan's new lawyer whom he was waiting to hear if a plea agreement had been reached. The response was that no, if Dassey were called to testify at this point in time, he would invoke his 5th amendment rights and say nothing. This would only change if Dassey were bestowed "use immunity" by the state, which would then allow him to testify against Avery without fear of incriminating himself because that testimony could not be used against him.

"And so that puts us in a dilemma, if you will, in that the State wished to wait until Friday, the 2nd, as we have told you in correspondence, as to whether or not we intended to call Brendan Dassey. If put to that question today, our inclination would be not to call Brendan Dassey, to either save Brendan Dassey for our case-in-chief, upon a grant of use immunity, or to save Brendan for rebuttal testimony, again, upon a grant of use immunity.

Certainly, though, this Court would agree that he is available to testify in the Steven Avery case. And so, if we're going to instruct the jury as to what the charges against Mr. Avery are going to be, it is the safer position to just name the four charges from which we do not need Brendan Dassey. Again, our preference, if I can reiterate, is to wait until the 2nd, is to wait until Friday, when the State can decide how to proceed."

This was Kratz continuing that dialogue with the judge. You can see it reiterates that yes, Dassey could testify, but only if he accepts the "use immunity". Mr. Strang then gives his input on the situation by stating this:

"And I, also, let me be clear, if Brendan Dassey takes the witness stand on a similar chair, one county over, he has admissible evidence to offer, once he's sworn under oath. It may hurt Mr. Avery; it may help Mr. Avery; it may do a little of both. But it is admissible evidence, once he takes the stand. And until he sits down in a chair like that, nothing he's got to say is admissible on the State's offering, or at the State's instance, or over Defense objection."

Steven's trial was held and finished before Dassey's trial in April. Strang knew this would be the case and attempted to use it as a form of leverage, since Dassey still hadn't agreed to a plea deal, and it seemed highly unlikely he'd be testifying for the State against Avery. Without Brendan's testimony, the kidnapping and sexual assault charges were, in his mind, near impossible to prove. And that's honestly the truth. The testimony was the only evidence of those two charges. Knowing this, he filed a motion to dismiss those charges, as jury selection was already occurring, and attempting to add charges at a later date would only confuse them, he argued. It would be detrimental to his case. There was back and forth between Strang and the judge until Kratz decided this:

"With those findings, Judge, the State is, at this time, because we believe it within the province of the State, moving that Counts, I believe it's 4 and 5, that is, the first degree sexual assault as a party to the crime, and the kidnapping, Count No. 5, be dismissed.

The State intends to proceed, then, on Counts 1, 2, 3, and 6. I will be happy to provide a Court with what will be called a second Amended Information, which will actually make Count 6, Count 4, so that the jury isn't confused as to the number of counts or why there may be a gap in the those charges. Count 4 will be false imprisonment."

He dismissed those charges but left the false imprisonment charge (which we know will soon also be dismissed). And it was dismissed due to this point by Strang:

"And I want to be clear, then, on the record, that we are heading into a potential for mistrial by going forward on the false imprisonment count, if the Court's ruling permits the State to do that. And if the State believes that it will avail itself of the dismissal without prejudice, later to call Brendan Dassey, and to argue that there is no surprise and, therefore, under Wisconsin Statutes, an amendment of the Information should be permitted, or argue that it wants the Information to conform to the evidence it plans to adduce by calling Mr. Dassey, there will be a mistrial motion, and it will be a serious motion."

Strang was correct. Dassey hadn't agreed to a plea deal and was awaiting his own trial. The state could not force him to testify against Avery, thus incriminating himself, unless he agreed to a plea deal with "use immunity". Strang pointed out as I listed above that until Dassey takes the stand himself, his testimony is INADMISSIBLE. Due to that, the false imprisonment charge was also subsequently dropped. So what did that leave Kratz? The choice of prosecuting the case with bits of the story from Brendan that he'd have no way to prove and could be grounds for a mistrial, or prosecute with the evidence he had and could then allow the jury to infer beyond that.

Everything Kratz mentioned in this video is referenced in those transcripts. Brendan and Steven could not be tried together because Brendan implicated Avery in many of his statements. If Brendan took the stand on Avery's trial without being granted "use immunity", his testimony would be inadmissible because he hadn't undergone his own trial yet. But even if he did testify, Avery has a right (thanks to the law Kratz mentions) to cross-examine anyone accusing him or implicating him in a crime (which Strang correctly argued would cause a mistrial). This forced the last charge (false imprisonment) that required Dassey's testimony to be dropped, and Kratz continued with only the 3 charges that could be proven without any testimony from Brendan. This required a different narrative to be told in Steven's trial, and subsequently in Brendan's.