That's what the Crown says in its opening statement, but I think that needs to be taken as a statement in legal terms, not as a description of who did what in the car. The Crown intends to prove that both accused are guilty of first-degree murder in the shooting of TB in his truck etc.
We've had posts earlier about what is needed for a murder to be considered first-degree. Intention and premeditation are generally required, but if a murder (or accidental death for that matter) results from kidnapping and/or forcible confinement, that is automatically first degree murder (ditto if the death results from sexual assault but that is not relevant to this case).
So, if it is satisfactorily proven that both accused forcibly abducted and confined TB, and did it together (i,e, in cahoots with each other), it doesn't require both to be in the truck when the shooting occurred. The one driving the Yukon would be just as guilty of the murder, because he is/was a party to the abduction and forcible confinement, as if he were in the truck simultaneously firing with a second gun.
IOW, the Crown is saying, TB was shot in the truck and both accused did it. Meaning, both accused are responsible and had the mens rea (the legal term -- the intent and mental framework) to commit the crime, as evidenced by the fact they set up the meeting, under duplicitous conditions, lied to TB and his wife and boarder about being "dropped off" and got into the truck with TB, effecting an abduction and forcible confinement according to a preconceived plan. What followed is the responsibility of both even if one was not in the truck when the crime occurred.
The Crown may not be able to prove who fired the fatal shot. Each lawyer may try to suggest that the other defendant did it. But even if either defendant could prove that to be true, it would not get him off the hook. Because he was a party to abduction and forcible confinement of TB, he is guilty of first-degree murder even if he didn't intend TB's death.
The part I don't get is, how did they subdue TB when MS presumably got out of the truck to drive the Yukon? I can't imagine him not trying to do something, whether grab the keys or get away or what. The video evidence from Super Sucker suggests that they wasted no time at the hayfield so the actual shooting must have happened later (besides which, Bullman the neighbour didn't report hearing gunshots).